* Introduce `functioncache` and `functioncontainer`

- introduce `functioncache` for managing function dependencies (e.g. setting up function class loader and its classpath). The `functioncache` is shared across multiple instances.
- introduce `functioncontainer` for providing a runtime environment to invoke functions.
- provide a thread based `functioncontainer` implementation, which uses functioncache to load classpath and initial its function class loader.

* Revert FunctionID/InstanceID to String

* Add test cases for functioncache and functioncontainer

### Motivation

The `AckGroupingTrackerEnabled`'s timer callback only captures `this`, which is a weak reference to the `AckGroupingTrackerEnabled ` instance. If the instance went out of the scope and destroyed, `this` would point to an invalid block.

Even if the destructor of `AckGroupingTrackerEnabled` cancels the timer, the callback may not be triggered immediately. There's still a possibility that when the callback is triggered, the error code is 0 but accessing to `this` is invalid. For example, there's a crash caused by the callback in production environment that is hard to reproduce:

```
#6 <signal handler called>
#7 0x00007fb4e67c5cb8 in ?? ()
#8 0x00007fb604981adb in operator() (ec=..., __closure=0x7fb52b0fb230)
   at /usr/local/src/apache-pulsar-microfocus/pulsar-client-cpp/lib/AckGroupingTrackerEnabled.cc:148
#9 operator() (this=0x7fb52b0fb230) at /usr/local/include/boost/asio/detail/bind_handler.hpp:47
```

### Modifications

- Use `std::shared_ptr` instead of `std::unique_ptr` for `AckGroupingTrackerEnabled`, then capture the shared pointer in timer callback's lambda expression to extend the lifetime of `this`.
- Add `start()` method to `AckGroupingTracker` to avoid `std::bad_weak_ptr` because `shared_from_this()` in a constructor returns a null pointer.
- Use `std::weak_ptr` to reference `HandlerBase` in case that the handler may be invalid when the timer callback is triggered.

### Motivation

The `AckGroupingTrackerEnabled`'s timer callback only captures `this`, which is a weak reference to the `AckGroupingTrackerEnabled ` instance. If the instance went out of the scope and destroyed, `this` would point to an invalid block.

Even if the destructor of `AckGroupingTrackerEnabled` cancels the timer, the callback may not be triggered immediately. There's still a possibility that when the callback is triggered, the error code is 0 but accessing to `this` is invalid. For example, there's a crash caused by the callback in production environment that is hard to reproduce:

```
#6 <signal handler called>
#7 0x00007fb4e67c5cb8 in ?? ()
#8 0x00007fb604981adb in operator() (ec=..., __closure=0x7fb52b0fb230)
   at /usr/local/src/apache-pulsar-microfocus/pulsar-client-cpp/lib/AckGroupingTrackerEnabled.cc:148
#9 operator() (this=0x7fb52b0fb230) at /usr/local/include/boost/asio/detail/bind_handler.hpp:47
```

### Modifications

- Use `std::shared_ptr` instead of `std::unique_ptr` for `AckGroupingTrackerEnabled`, then capture the shared pointer in timer callback's lambda expression to extend the lifetime of `this`.
- Add `start()` method to `AckGroupingTracker` to avoid `std::bad_weak_ptr` because `shared_from_this()` in a constructor returns a null pointer.
- Use `std::weak_ptr` to reference `HandlerBase` in case that the handler may be invalid when the timer callback is triggered.

### Motivation

The `AckGroupingTrackerEnabled`'s timer callback only captures `this`, which is a weak reference to the `AckGroupingTrackerEnabled ` instance. If the instance went out of the scope and destroyed, `this` would point to an invalid block.

Even if the destructor of `AckGroupingTrackerEnabled` cancels the timer, the callback may not be triggered immediately. There's still a possibility that when the callback is triggered, the error code is 0 but accessing to `this` is invalid. For example, there's a crash caused by the callback in production environment that is hard to reproduce:

```
#6 <signal handler called>
#7 0x00007fb4e67c5cb8 in ?? ()
#8 0x00007fb604981adb in operator() (ec=..., __closure=0x7fb52b0fb230)
   at /usr/local/src/apache-pulsar-microfocus/pulsar-client-cpp/lib/AckGroupingTrackerEnabled.cc:148
#9 operator() (this=0x7fb52b0fb230) at /usr/local/include/boost/asio/detail/bind_handler.hpp:47
```

### Modifications

- Use `std::shared_ptr` instead of `std::unique_ptr` for `AckGroupingTrackerEnabled`, then capture the shared pointer in timer callback's lambda expression to extend the lifetime of `this`.
- Add `start()` method to `AckGroupingTracker` to avoid `std::bad_weak_ptr` because `shared_from_this()` in a constructor returns a null pointer.
- Use `std::weak_ptr` to reference `HandlerBase` in case that the handler may be invalid when the timer callback is triggered.

(cherry picked from commit cfa65d0)

)

### Motivation

The `AckGroupingTrackerEnabled`'s timer callback only captures `this`, which is a weak reference to the `AckGroupingTrackerEnabled ` instance. If the instance went out of the scope and destroyed, `this` would point to an invalid block.

Even if the destructor of `AckGroupingTrackerEnabled` cancels the timer, the callback may not be triggered immediately. There's still a possibility that when the callback is triggered, the error code is 0 but accessing to `this` is invalid. For example, there's a crash caused by the callback in production environment that is hard to reproduce:

```
#6 <signal handler called>
apache#7 0x00007fb4e67c5cb8 in ?? ()
apache#8 0x00007fb604981adb in operator() (ec=..., __closure=0x7fb52b0fb230)
   at /usr/local/src/apache-pulsar-microfocus/pulsar-client-cpp/lib/AckGroupingTrackerEnabled.cc:148
apache#9 operator() (this=0x7fb52b0fb230) at /usr/local/include/boost/asio/detail/bind_handler.hpp:47
```

### Modifications

- Use `std::shared_ptr` instead of `std::unique_ptr` for `AckGroupingTrackerEnabled`, then capture the shared pointer in timer callback's lambda expression to extend the lifetime of `this`.
- Add `start()` method to `AckGroupingTracker` to avoid `std::bad_weak_ptr` because `shared_from_this()` in a constructor returns a null pointer.
- Use `std::weak_ptr` to reference `HandlerBase` in case that the handler may be invalid when the timer callback is triggered.