func_lock: fix multiple-channel-grant problems.

Under contention it becomes possible that multiple channels will be told
they successfully obtained the lock, which is a bug.  Please refer

ASTERISK-29217

This introduces a couple of changes.

1.  Replaces requesters ao2 container with simple counter (we don't
    really care who is waiting for the lock, only how many).  This is
    updated undex ->mutex to prevent memory access races.
2.  Correct semantics for ast_cond_timedwait() as described in
    pthread_cond_broadcast(3P) is used (multiple threads can be released
    on a single _signal()).
3.  Module unload races are taken care of and memory properly cleaned
    up.

Change-Id: I6f68b5ec82ff25b2909daf6e4d19ca864a463e29
Signed-off-by: Jaco Kroon <jaco@uls.co.za>