taskpool: Add taskpool API, switch Stasis to using it.

This change introduces a new API called taskpool. This is a pool
of taskprocessors. It provides the following functionality:

1. Task pushing to a pool of taskprocessors
2. Synchronous tasks
3. Serializers for execution ordering of tasks
4. Growing/shrinking of number of taskprocessors in pool

This functionality already exists through the combination of
threadpool+taskprocessors but through investigating I determined
that this carries substantial overhead for short to medium duration
tasks. The threadpool uses a single queue of work, and for management
of threads it involves additional tasks.

I wrote taskpool to eliminate the extra overhead and management
as much as possible. Instead of a single queue of work each
taskprocessor has its own queue and at push time a selector chooses
the taskprocessor to queue the task to. Each taskprocessor also
has its own thread like normal. This spreads out the tasks immediately
and reduces contention on shared resources.

Using the included efficiency tests the number of tasks that can be
executed per second in a taskpool is 6-12 times more than an equivalent
threadpool+taskprocessor setup.

Stasis has been moved over to using this new API as it is a heavy consumer
of threadpool+taskprocessors and produces a lot of tasks.

UpgradeNote: The threadpool_* options in stasis.conf have now been deprecated
though they continue to be read and used. They have been replaced with taskpool
options that give greater control over the underlying taskpool used for stasis.

DeveloperNote: The taskpool API has been added for common usage of a
pool of taskprocessors. It is suggested to use this API instead of the
threadpool+taskprocessor approach.

main/taskprocessor.c

@@ -967,6 +967,11 @@ long ast_taskprocessor_size(struct ast_taskprocessor *tps)
 	return (tps) ? tps->tps_queue_size : -1;
 }
 
+struct ast_taskprocessor_listener *ast_taskprocessor_listener(struct ast_taskprocessor *tps)
+{
+	return tps ? tps->listener : NULL;
+}
+
 /* taskprocessor name accessor */
 const char *ast_taskprocessor_name(struct ast_taskprocessor *tps)
 {