### Motivation:

PR #3297 is changing `inEventLoop` and `inEventLoop` is very much on the
perf path. So let's add a benchmark

### Modifications:

- Add `(!)inEventLoop` benchmark
- Fix a guaranteed deadlock in `TCPThroughputBenchmark` to make the
benchmarks runnable

### Result:

- More benchmarks
- Fewer hangs

### Motivation:

`inEventLoop` is very much in the performance path of SwiftNIO,
especially these days with Concurrency, `NIOLoopBound` and friends.
Previously, we relied on `pthread_equal(pthread_self(), myPthread)`,
however, this could cause a number of issues.

1. Holding onto a `pthread_t` after `.join` is actually illegal (fixed
in #3297)
2. Potential ABA issues when `pthread_t` pointer are re-used for new
pthreads
3. Fix would require a lock around `myPthread` which makes things (2x
slower, even without contention)

### Modifications:

- New type `SelectableEventLoopUniqueID` which can be packed into a
`UInt`
- Attach them into a C thread local

### Result:

- Even faster than the old, incorrect version
- old: `measuring: el_in_eventloop_100M: 0.257395375, 0.241049208,
0.243188792, 0.259125916, 0.24843225, 0.229690125, 0.244281541,
0.225078834, 0.236395, 0.233305167`
- new: `measuring: el_in_eventloop_100M: 0.175561125, 0.187225625,
0.199269375, 0.19740975, 0.1922695, 0.179850958, 0.177612458,
0.17665125, 0.17897475, 0.18038775`
- More correct
- Groundwork to make #3297 not make things slower

### Motivation:

Structured Concurrency is helpful but so far, MTELG was very difficult
to use with it mostly because Swift is still missing `async` in
`defer`s.

This builds on
- #3297 
- #3302
- #3304

### Modifications:

- Add MTELG.withELG { ... }

### Result:

MTELG + SC = <3

### Motivation:

`inEventLoop` is very much in the performance path of SwiftNIO,
especially these days with Concurrency, `NIOLoopBound` and friends.
Previously, we relied on `pthread_equal(pthread_self(), myPthread)`,
however, this could cause a number of issues.

1. Holding onto a `pthread_t` after `.join` is actually illegal (fixed
in apple#3297)
2. Potential ABA issues when `pthread_t` pointer are re-used for new
pthreads
3. Fix would require a lock around `myPthread` which makes things (2x
slower, even without contention)

### Modifications:

- New type `SelectableEventLoopUniqueID` which can be packed into a
`UInt`
- Attach them into a C thread local

### Result:

- Even faster than the old, incorrect version
- old: `measuring: el_in_eventloop_100M: 0.257395375, 0.241049208,
0.243188792, 0.259125916, 0.24843225, 0.229690125, 0.244281541,
0.225078834, 0.236395, 0.233305167`
- new: `measuring: el_in_eventloop_100M: 0.175561125, 0.187225625,
0.199269375, 0.19740975, 0.1922695, 0.179850958, 0.177612458,
0.17665125, 0.17897475, 0.18038775`
- More correct
- Groundwork to make apple#3297 not make things slower

### Motivation:

NIOThread assumed it can hold onto `pthread_t` forever. Unfortunately,
on Linux, `pthread_t` is dead when the thread got joined (if not
detached) or if it exits (if detached) _which is at a random point in
time_.

### Modifications:

- Only assume ownership owner the `pthread_t` if not detached
- `nil` out the `pthread_t` on `join()`

### Result:

- Fewer `pthread_t` lifecycle bugs.

### Motivation:

Structured Concurrency is helpful but so far, MTELG was very difficult
to use with it mostly because Swift is still missing `async` in
`defer`s.

This builds on
- apple#3297 
- apple#3302
- apple#3304

### Modifications:

- Add MTELG.withELG { ... }

### Result:

MTELG + SC = <3

…3360)

### Motivation:

In #3297, I introduced a deadlock involving `SelectableEventLoop`'s
`debugDescription`. I was under the impression that it's not possible to
call this from outside the `NIO` module so I deemed it safe. That was a
mistake :).

### Modifications:

- Make it impossible to deadlock around
`SelectableEventLoop.debugDescription`.

### Result:

- Fewer deadlocks