Pre-allocate transmission data buffer for RLS NOTIFY requests.

Pre-allocate transmission data buffer for RLS NOTIFY requests.

PJSIP, unless a constant is modified at compilation time, limits
SIP requests to 4000 bytes. Full-state RLS notifications can easily
exceed this limit with moderately small lists.

This changeset allows for Asterisk to work around this size limit by
performing its own allocation of the transmission data buffer. This
way, Asterisk can allocate a buffer that exceeds the built-in maximum.

We still impose our own limit of 64000 bytes, mainly because making
allocations larger than that is a bit absurd.

ASTERISK-24181 #close
Reported by Mark Michelson

Review: https://reviewboard.asterisk.org/r/3977
........

Merged revisions 422851 from http://svn.asterisk.org/svn/asterisk/branches/13

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@422852 65c4cc65-6c06-0410-ace0-fbb531ad65f3