MEDIUM: listeners: don't bounce listeners management between queues

During 2.1 development, commit f2cb16948 ("BUG/MAJOR: listener: fix
thread safety in resume_listener()") was introduced to bounce the
enabling/disabling of a listener's FD to one of its threads because
the remains of fd_update_cache() were fundamentally incompatible with
the need to call fd_want_recv() or fd_stop_recv() for another thread.

However since then we've totally dropped such code and it's totally
safe to use these functions on an FD that is solely used by another
thread (this is even used by the FD migration code). The only remaining
limitation concerning the wake up delay was addressed by previous commit
"MEDIUM: fd: always wake up one thread when enabling a foreing FD".

The current situation forces the FD management to remain in the
pause_listener() and resume_listener() functions just so that it can
bounce between threads, without having the ability to delegate it to
the suitable protocol layer.

So let's first remove this now unneeded workaround.
1 file changed