Add the average talk time for a queue

This patch adds the functionality to app_queue of calculating
the average amount of time that channels are bridged for a
queue. The algorithm used to calculate the average is the same
exponential average currently used to calculate the average holdtime.
See the CHANGES file to see the methods you may use to view this
information.

(closes issue #13960)
Reported by: coolmig
Patches:
      app_queue.c.diff.trunk-r158840 uploaded by coolmig (license 621)



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@167792 65c4cc65-6c06-0410-ace0-fbb531ad65f3