Merge changes from team/bbryant/keyrotation

This set of changes enhances IAX2 encryption support by adding key rotation
to provide enhanced security.  The key used for encryption is rotated right 
after the call gets set up, and then again every few minutes.  This was
discussed at the last AstriDevCon.  For interoperability with older versions
of Asterisk, there is an option that disables key rotation.

(closes issue #13018)
Reported by: bbryant
Patches:
      07072008__iax2_key_rotation.diff uploaded by bbryant (license 36)
Tested by: russell, bbryant


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@135158 65c4cc65-6c06-0410-ace0-fbb531ad65f3