PT48390337 - SQA error log messages in sipXproxy.log

SQA clients and server logged various error messages because client
connections to server were closed by server when error conditions
where hit in the socket parsing func  which was buggy. The server side
parsing of read buffer was fixed: parsing will be started only when
min message size was read in buffer, expect that several reads might
not give a full message, preserve parsing state between subsequent socket
reads, refactored the parsing func completely because the old design was
using two functions which in rare cases would get stuck in infinite recursion.
Other fixes:
Added a flag to mark that a socket was closed so that subsequent attempts
of closing will be discarded. This also resulted in lots of benign error logs.
Removed duplicate code that closed a socket from various locations: connection
class, listener class.
Check if read error is EOF to know that connection was closed gracefully.
Merge branch 'PT48390337' into release-4.6-unstable-18