FreeSWITCH

task scheduler: Wait on a condition variable rather than queue.

Rather than have the worker thread block on a queue of events, have it
block waiting on a condition variable. This design has three advantages.

Firstly, as not all events which require the worker thread to spin loop
enqueue events (task deleted, task completed) the loop is set to spin
twice a second to pick up such events and provide the required task
scheduling time granularity. By having a condition variable, these
events can spin the loop, allowing the loop to wait for possibly longer
times. This is the subject of a future change.

Secondly, this change makes locking easier, allowing the worker thread
to possess the mutex, relinquishing it only when waiting or executing.

Thirdly, this change drops the requirement for
switch_queue_pop_timeout(), a function which depends on out-of-tree
features of apr-utils.