…rService

Fixes apache#22569

### Motivation

`BrokerService#closeAsync` calls `unloadNamespaceBundlesGracefully` to
unload namespaces gracefully. With extensible load manager, it
eventually calls `TableViewLoadDataStoreImpl#validateProducer`:

```
BrokerService#unloadNamespaceBundlesGracefully
  ExtensibleLoadManagerWrapper#disableBroker
    ExtensibleLoadManagerImpl#disableBroker
      ServiceUnitStateChannelImpl#cleanOwnerships
        ServiceUnitStateChannelImpl#doCleanup
          TableViewLoadDataStoreImpl#removeAsync
            TableViewLoadDataStoreImpl#validateProducer
```

In `validateProducer`, if the producer is not connected, it will
recreate the producer synchronously. However, since the state of
`PulsarService` has already been changed to `Closing`, all connect or
lookup requests will fail with `ServiceNotReady`. Then the client will
retry until timeout.

Besides, the unload operation could also trigger the reconnection
because the extensible load manager sends the unload event to the
`loadbalancer-service-unit-state` topic.

### Modifications

The major fix:
Before changing PulsarService's state to `Closing`, call
`BrokerService#unloadNamespaceBundlesGracefully` first to make the load
manager complete the unload operations first.

Minor fixes:
- Record the time when `LoadManager#disableBroker` is done.
- Don't check if producer is disconnected because the producer could
  retry if it's disconnected.

### Verifications

Add `ExtensibleLoadManagerCloseTest` to verify closing `PulsarService`
won't take too much time. Here are some test results locally:

```
2024-04-24T19:43:38,851 - INFO  - [main:ExtensibleLoadManagerCloseTest] - Brokers close time: [3342, 3276, 3310]
2024-04-24T19:44:26,711 - INFO  - [main:ExtensibleLoadManagerCloseTest] - Brokers close time: [3357, 3258, 3298]
2024-04-24T19:46:16,791 - INFO  - [main:ExtensibleLoadManagerCloseTest] - Brokers close time: [3313, 3257, 3263]
2024-04-24T20:13:05,763 - INFO  - [main:ExtensibleLoadManagerCloseTest] - Brokers close time: [3304, 3279, 3299]
2024-04-24T20:13:43,979 - INFO  - [main:ExtensibleLoadManagerCloseTest] - Brokers close time: [3343, 3308, 3310]
```

As you can see, each broker takes only about 3 seconds to close due to
`OWNERSHIP_CLEAN_UP_CONVERGENCE_DELAY_IN_MILLIS` value added in
apache#20315