MEDIUM: tasks/fd: replace sleeping_thread_mask with a TH_FL_SLEEPING flag

MEDIUM: tasks/fd: replace sleeping_thread_mask with a TH_FL_SLEEPING flag

Every single place where sleeping_thread_mask was still used was to test
or set a single thread. We can now add a per-thread flag to indicate a
thread is sleeping, and remove this shared mask.

The wake_thread() function now always performs an atomic fetch-and-or
instead of a first load then an atomic OR. That's cleaner and more
reliable.

This is not easy to test, as broadcast FD events are rare. The good
way to test for this is to run a very low rate-limited frontend with
a listener that listens to the fewest possible threads (2), and to
send it only 1 connection at a time. The listener will periodically
pause and the wakeup task will sometimes wake up on a random thread
and will call wake_thread():

   frontend test
        bind :8888 maxconn 10 thread 1-2
        rate-limit sessions 5

Alternately, disabling/enabling a frontend in loops via the CLI also
broadcasts such events, but they're more difficult to observe since
this is causing connection failures.