Retain a ref to NIOAsyncWriter until channel active (#2703)

Motivation:

NIOAsyncChannel requires users to explicitly close it, this is typically
done by calling `executeThenClose`. If a `NIOAsyncChannel` isn't closed
then its outbound writer will hit a precondition failure on `deinit`.
Not calling `executeThenClose` is a programmer error.

However there are some sharp edges: if NIO never returns the
`NIOAsyncChannel` to the caller (e.g. if a connect attempt fails) then
nothing will finish the writer and precondition will fail in the deinit.
Working around this from a user perspective is non-obvious and requires
keep tracking of all `NIOAsyncChannel`s created from a connect attempt
and closing the unused ones.

We still want to maintain the precondition when users don't close the
channel, one way of achieving this is by defining a point in time at
which NIO hands responsibility of the channel to the user.

Modifications:

- Retain the writer in the outbound writer handler until channel active
- On successful connect attempts, the channel becomes active and the
  connected channel is returned to the caller.
- On failed attempts channel active isn't called so the writer is
  retained until the handler is removed from the pipeline at which point it is
  finished.

Result:

Failed connect attempts don't result in precondition failures when using
NIOAsyncChannel.

Fix memory allocations counters on macOS. (#2673)

Motivation:

Allocation counters are broken on macOS as described [here](#2672). This PR fixes it.

Modifications:

`AtomicCounter` library is made `.dynamic`. This deduplicates the two (static) copies that used to exist. One embedded in the main binary, and the other in `HookedFunctions` dylib. This deplication fixes the issue, because there is just one copy of the counters in the process address space.

Result:

All the tests counting allocations and related statistics work on macOS.

no confstr on Android (#2627)

Fix the typed HTTP upgrade compiler guards (#2594)

The compiler guards were unnecessarily complex and I also didn't cover 3 methods where we used the types.

use feature-specific guard for @retroactive (#2581)

Motivation:

We should use the more granular guard for uses of @retroactive rather
than coarse swift versions to guard against corner-cases

Modifications:

Switch `#if compiler(>=5.11)` for `#if hasFeature(RetroactiveAttribute)`

Result:

No change in most cases, more protected against corner-cases.

Fix exclusive access violation in `NIOAsyncChannelOutboundWriterHandl…

…er` (#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 (#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 (#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 (#2497)

# Motivation

Fixes #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 (#2466)