res_rtp_asterisk: Add support for DTLS handshake retransmissions

On congested networks, it is possible for the DTLS handshake messages to get
lost. This patch adds a timer to res_rtp_asterisk that will periodically
check to see if the handshake has succeeded. If not, it will retransmit the
DTLS handshake.

Review: https://reviewboard.asterisk.org/r/3337

ASTERISK-23649 #close
Reported by: Nitesh Bansal
patches:
  dtls_retransmission.patch uploaded by Nitesh Bansal (License 6418)
........

Merged revisions 413008 from http://svn.asterisk.org/svn/asterisk/branches/11
........

Merged revisions 413009 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@413012 65c4cc65-6c06-0410-ace0-fbb531ad65f3