channels/chan_iax2: Improve POKE expiration time calculation for lossy networks

POKE is used to check for peer availability; however, in networks with packet
loss, the current calculations may result in POKE expiration times that are too
short. This patch alters the expiration/retry time logic to take into account
the last known qualify round trip time, as opposed to always using a static
value for each peer.

Review: https://reviewboard.asterisk.org/r/4536

ASTERISK-22352 #close
Reported by: Frederic Van Espen

ASTERISK-24894 #close
Reported by: Y Ateya
patches:
  poke_noanswer_duration.diff submitted by Y Ateya (License 6693)
........

Merged revisions 434564 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@434565 65c4cc65-6c06-0410-ace0-fbb531ad65f3