Update the timing API to have better support for multiple timing interfaces.

1) Add module use count handling so that timing modules can be unloaded.

2) Implement unload_module() functions for the timing interface modules.

3) Allow multiple timing modules to be loaded, and use the one with the
   highest priority value.

4) Report which timing module is being use in the "timing test" CLI command.

(closes issue #14489)
Reported by: russell

Review: http://reviewboard.digium.com/r/162/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@176666 65c4cc65-6c06-0410-ace0-fbb531ad65f3