Ensure single reader and writer to system fd on Unix

[ysbaddaden]

This patch extends the fdlock to serialize reads and writes by extending the reference counted lock with a read lock and a write lock, so taking a reference and locking acts as a single operation instead of two (1. acquire/release the lock; 2. take/return a reference). This avoids a race condition in the polling event loops:

• Fiber 1 then Fiber 2 try to read from fd;
• Since fd isn't ready, both fibers start waiting;
• When fd becomes ready then Fiber 1 is resumed;
• Fiber 1 doesn't read everything and returns;
• Since events are edge-triggered, Fiber 2 won't be resumed!!!

With the read lock, fiber 2 will wait on the lock then be resumed by fiber 1 when it returns. A concrete example is multiple fibers waiting to accept on a socket where fiber 1 would keep handling connections, while fiber 2 sits idle.

The other benefit is that it can help to simplify the evloops that will now only deal with a single reader + single writer per IO and is required for the io_uring evloop (the MT version requires it).

NOTE: While this patch only serializes reads/writes on UNIX at the Crystal::System, which is where the bugs are, we may want to move will move it into stdlib for all targets at some point, for example to serialize reads and writes around IO::Buffered. See #16289 (comment)

Depends on #16288 and #16289.
Required by #16264.

[ysbaddaden]

I split the fdlock in two different commits (refcount then serial R/W) that outline the different steps for merging as individual PRs.

[ysbaddaden]

Rebased from master to bring #16288 and #16289 that this patch depends on.

[ysbaddaden]

Usages are still restricted to Crystal::System types on UNIX because trying to move it out requires to implement IOCP#shutdown that itself needs the single reader and single writer locks that this PR brings (chicken-egg issue 🐣) for reasons explained in #16289 (comment).

I'll prepare a third PR that will:

• Implement Crystal::EventLoop::IOCP#shutdown(Socket);
• Implement Crystal::EventLoop::IOCP#shutdown(IO::FileDescriptor);
• Move @fd_lock to IO::FileDescriptor and Socket.

[straight-shoota]

🎉