If thread A is about to wake as thread B is about to sleep, and A loses
the GIL at an inopportune moment, it was possible for two latches to
share the same socketpair, causing wakeups routed to the wrong latch.
The pair was returned to the 'idle sockets' list before .recv() had been
called. This manifested as TimeoutError() thrown rarely with many active
threads and the host is heavily loaded (such as Travis CI).
Add more documentation and stop writing single wake bytes. Instead the
recipient's identity is written instead, making it simpler to detect
future bugs.
484d4fdb74