Adds cdr logging of calls resulting in CONGESTION

Applies a patch made a long time ago by alecdavis which adds a CDR feature for logging
calls that failed due to congestion.

(closes issue #15907)
Reported by: alecdavis
Patches: 
      cdr_congestion.diff.txt uploaded by alecdavis (license #5546)

Review: https://reviewboard.asterisk.org/r/454/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@329835 65c4cc65-6c06-0410-ace0-fbb531ad65f3