res_pjsip_transport_websocket: Fix security events and simplify implementation.

Transport type determination for security events has been simplified to use
the type present on the message itself instead of searching through configured
transports to find the transport used.

The actual WebSocket transport has also been simplified. It now leverages the
existing PJSIP transport manager for finding the active WebSocket transport
for outgoing messages. This removes the need for res_pjsip_transport_websocket
to store a mapping itself.

(closes issue ASTERISK-22897)
Reported by: Max E. Reyes Vera J.

Review: https://reviewboard.asterisk.org/r/3036/ 
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Merged revisions 403256 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@403257 65c4cc65-6c06-0410-ace0-fbb531ad65f3