channels/chan_iax2: Add a configuration parameter for call token expiration

This patch adds a new configuration parameter, 'calltokenexpiration', that
controls how long before an authentication call token is expired. The default
maintains the RFC specified 10 seconds. Setting it to a higher value may be
useful in lossy networks.

Review: https://reviewboard.asterisk.org/r/4588

ASTERISK-24939 #close
Reported by: Y Ateya
patches:
  ctoken_configuration.diff submitted by Y Ateya (License 6693)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@434563 65c4cc65-6c06-0410-ace0-fbb531ad65f3