Resolve some synchronization issues in chan_iax2 scheduler handling.

Resolve some synchronization issues in chan_iax2 scheduler handling.

The important changes here are related to the synchronization between threads
adding items into the scheduler and the scheduler handling thread.  By adjusting
the lock and condition handling, we ensure that the scheduler thread sleeps no
longer and no less than it is supposed to.  We also ensure that it does not
wake up more often than it has to.

There is no bug report associated with this.  It is just something that I found
while putting scheduler thread handling into a reusable form (review 129).

Review: http://reviewboard.digium.com/r/131/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@171452 65c4cc65-6c06-0410-ace0-fbb531ad65f3