FreeSWITCH

task scheduler: Wait on tasks to exit rather than sleep in loop.

Revision of switch_scheduler_task_thread_stop() to wait on the condition
variable for all tasks to complete rather than simply spinning in a loop
with sleeps.

This change also moves the deallocation into
switch_scheduler_task_thread_stop(). The reason for the change is that
the task_thread may be stuck in the middle of a hung task and cannot
delete tasks on its own, or the task_thread may have exited before any
tasks in their own thread, so we need to clean them up.

Also add checks to add_task() and del_task() functions to check that the
task thread is running, and have switch_scheduler_task_thread_start()
block until the task thread is indeed started.