feat: use double-buffer for winapi observer

[nascheme]

This improves the performance of the winapi observer by using a second thread and by using double-buffering.

The read_directory_changes() function was replaced with a class DirectoryChangeReader() that takes care of reading the Windows handle. The ReadDirectoryChangesW() calls are made in a separate background thread and data returned is copied into a separate queue, to be processed with _parse_event_buffer() in a separate thread.

The time window for events to be lost due to processing outside the ReadDirectoryChangesW() call should be smaller. The events buffer is reused rather than creating a new one on every call.

The management of the Windows handle is encapulated in the DirectoryChangeReader() class and it takes care of waking the background thread, cleanly closing the handle and cleaning up the thread.

This partially addresses GH-1019. However, a better fix would use either overlapped IO or completion ports. However, that would be a more complex change.

[BoboTiG]

Thanks @nascheme!

I guess there is at least one test to adapt on Windows, cf the failure(s) on the CI.

👍 for me when the CI will be green (take care of Windows only, we already know the Linux & macOS tests are passing).

[nascheme]

Should be fixed with my latest commit (need a short sleep before ending the test__init__ case so that the background thread can cleanly exit).