Merge changes from timing branch

 - Convert chan_iax2 to use the timing API
 - Convert usage of timing in the core to use the timing API instead of
   using DAHDI directly
 - Make a change to the timing API to add the set_rate() function
 - change the timing core to use a rwlock
 - merge a timing implementation, res_timing_dahdi

Basic testing was successful using res_timing_dahdi


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@122523 65c4cc65-6c06-0410-ace0-fbb531ad65f3