res_pjsip_transport_websocket: Fix a progressive memory growth.

The packet structure used to receive messages was using the transport
pool. This meant that for each parsing the pool would grow accordingly.
Since memory can not be reclaimed without resetting it this would
cause the memory pool to grow and grow.

This change uses a specific memory pool for the packet structure and
resets it to a fresh state after the message has been received and
handled.
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Merged revisions 421939 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 421945 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@421950 65c4cc65-6c06-0410-ace0-fbb531ad65f3