Some scheduler API cleanup and improvements.

Previously, I had added the ast_sched_thread stuff that was a generic scheduler
thread implementation.  However, if you used it, it required using different
functions for modifying scheduler contents.  This patch reworks how this is
done and just allows you to optionally start a thread on the original scheduler
context structure that has always been there.  This makes it trivial to switch
to the generic scheduler thread implementation without having to touch any of
the other code that adds or removes scheduler entries.

In passing, I made some naming tweaks to add ast_ prefixes where they were not
there before.

Review: https://reviewboard.asterisk.org/r/1007/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@299091 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/sip/include/globals.h

@@ -25,7 +25,7 @@
 #define _SIP_GLOBALS_H
 
 extern struct ast_sockaddr bindaddr;     /*!< UDP: The address we bind to */
-extern struct sched_context *sched;     /*!< The scheduling context */
+extern struct ast_sched_context *sched;     /*!< The scheduling context */
 
 /*! \brief Definition of this channel for PBX channel registration */
 extern const struct ast_channel_tech sip_tech;