res_pjsip_keepalive: Add runtime configurable keepalive module for connection-oriente...

res_pjsip_keepalive: Add runtime configurable keepalive module for connection-oriented transports.

Note that this is backport from trunk of r425825.

This change adds a module which is configurable using the keep_alive_interval setting in the
global section that will send a CRLF keep alive to all active connection-oriented transports at
the provided interval. This is useful because it can help keep connections open through NATs.
This functionality also exists within PJSIP but can not be controlled at runtime and requires
recompiling it.

Review: https://reviewboard.asterisk.org/r/4084/

ASTERISK-24644 #close

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@430084 65c4cc65-6c06-0410-ace0-fbb531ad65f3