Taskprocessor optimization; switch Stasis to use taskprocessors

Taskprocessor optimization; switch Stasis to use taskprocessors

This patch optimizes taskprocessor to use a semaphore for signaling,
which the OS can do a better job at managing contention and waiting
that we can with a mutex and condition.

The taskprocessor execution was also slightly optimized to reduce the
number of locks taken.

The only observable difference in the taskprocessor implementation is
that when the final reference to the taskprocessor goes away, it will
execute all tasks to completion instead of discarding the unexecuted
tasks.

For systems where unnamed semaphores are not supported, a really
simple semaphore implementation is provided. (Which gives identical
performance as the original taskprocessor implementation).

The way we ended up implementing Stasis caused the threadpool to be a
burden instead of a boost to performance. This was switched to just
use taskprocessors directly for subscriptions.

Review: https://reviewboard.asterisk.org/r/2881/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@400178 65c4cc65-6c06-0410-ace0-fbb531ad65f3