Fix exclusive access violation in `NIOAsyncChannelOutboundWriterHandl…

…er` (apple#2580)

# Motivation
We were setting `self.sink = nil` in the `NIOAsyncChannelOutboundWriterHandler` twice in the same call stack which is an exclusivity violation. This happens because the first `self.sink = nil` triggers the `didTerminate` delegate call which again triggered `self.sink = nil`.

# Modification
This PR changes the code to only call `self.sink?.finish()` and only sets the `sink` to `nil` in the `didTerminate` implementation. This follows what we do for the inbound handler implementation. I also added a test that triggers this exclusivity violation.

# Result
No more exclusivity violations in our code.

Fix thread-safety issues in TCPThroughputBenchmark (apple#2537)

Motivation:

Several thread-safety issues were missed in code review. This patch fixes them.

Modifications:

- Removed the use of an unstructured Task, replaced with eventLoop.execute to
  ServerHandler's EventLoop.
- Stopped ClientHandler reaching into the benchmark object without any
  synchronization, used promises and event loop hops instead.

Result:

Thread safety is back

Support for custom protocols in DatagramBootstrap (apple#2516)

Motivation

This patch adds support for custom protocol families to
DatagramBootstrap. In most cases this isn't useful, and will fail, but
in many OSes it's a recognised system for non-root processes to get a
way to send ICMP echo requests. This is worth supporting.

Modifications

- Expose the `ProtocolSubtype` type publicly.
- Add a `protocolSubtype` setter on `DatagramBootstrap`
- Write a test that actually does ICMP as non-root

Results

Users can send ICMP echo requests without root privileges.

Avoid using deinit to fulfil the protocol negotiation promise (apple#…

…2497)

# Motivation

Fixes apple#2494

# Modification
This PR avoids using `deinit` to fulfil the protocol negotiation promise and opts to trap instead when it is being accessed before the handler is added. This allows us to use `handlerAdded` and `handlerRemoved`.

# Result
No more `deinit` usage that can be observed.

Make `NIOAsyncChannel.Configuration` Sendable (apple#2466)

Add configuration for `NIOAsyncChannel` (apple#2464)

# Motivation
While adopting the new `NIOAsyncChannel` type we saw an exploding number of parameters passed to the various connect/bind/configure methods. All these methods had in common that we had to pass configuration for the `NIOAsyncChannel`.

# Modification
This PR introduces a new type `NIOAsyncChannel.Configuration` which groups all four configuration parameters into a struct.

# Result
We can now write more concise methods that use a single configuration object.

Fix last remaining test on Android (apple#2457)

Motivation

Get all tests passing on Android AArch64

Modifications

- Check if localhost is set for IPv6 in BootstrapTest/testClientBindWorksOnSocketsBoundToEitherIPv4OrIPv6Only,
  and use ip6-localhost if not

Result

All tests pass natively on Android AArch64

Pooled control message storage. (apple#2422)

Tolerate sending data after close(mode: .output) (apple#2421)

Motivation

We shouldn't crash on somewhat likely user error.

Modifications

Pass on writes after close(mode: .output) instead of crashing.

Result

User code is more robust to weird edge cases.

Add unprocessedBytes property on NIOSingleStepByteToMessageProcessor (a…

…pple#2419)