[vm] Simplify implementation of native ports

This CL turns native ports into a thin abstraction over underlying
thread pool instead of building them as full fledged MessageHandler.

This allows to easily implement a variation of native ports which can
handle messages concurrently with the given degree of concurrency.
This type of port can be used to greatly simplify implementation of
IOService - which previously had to do its own concurrency management
on top of "single threaded" native ports. This capability is exposed
as `Dart_NewConcurrentNativePort` API.

The new implementation is in general much cleaner then the old one
with one exception: `Dart_CloseNativePort` API has unfortunate design
where underlying message handler is destroyed asynchronously and
`Dart_CloseNativePort` returns immediately without waiting for pending
tasks to complete. Implementing this on top of `ThreadPool` requires
some changes to thread pool implementation.

Issue https://github.com/dart-lang/sdk/issues/55844

TEST=ci

Change-Id: I062040ff233e93962ae93684e9b044d8facdaffc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/382163
Commit-Queue: Slava Egorov <vegorov@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>