Fix threadpool rapid growth problem.

When a threadpool is set to autoincrement its threadcount, an issue
may arise when multiple tasks are queued at once into the threadpool. Since
threads start active, each new task would result in autoincrementing the
thread count. So if all threads were active, and a thread's autoincrement
value were 5, then 3 new tasks would result in 15 threads being created even
though the initial autoincrement was sufficient to handle the number of tasks.

This change introduces three behavior changes:

1) New threads in the threadpool start idle instead of active.
2) When a threadpool autoincrements, one thread is activated after the growth.
3) When a threadpool's size is incremented manually, all added threads are activated.

For a more detailed explanation about the changes, please see the Review Board link
at the bottom of this commit.

Review: https://reviewboard.asterisk.org/r/2629



git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@392318 f38db490-d61c-443f-a65b-d21fe96a405b