Add a common implementation of a scheduler context with a dedicated thread.

This commit expands the Asterisk scheduler API to include a common implementation
of a scheduler context being processed by a dedicated thread.  chan_iax2 has been
updated to use this new code.  Also, as a result, this resolves some race
conditions related to the previous chan_iax2 scheduler handling.

Related to rev 171452 which resolved the same issues in 1.4.

Code from team/russell/sched_thread2

Review: http://reviewboard.digium.com/r/129/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@173858 65c4cc65-6c06-0410-ace0-fbb531ad65f3