SIP TCP/TLS: move client connection setup/write into tcp helper thread, various related locking/memory fixes.

        What this patch fixes
1.Moves sip TCP/TLS connection setup into the TCP helper thread:
  Connection setup takes awhile and before this it was being
  done while holding the monitor lock.
2.Moves TCP/TLS writing to the TCP helper thread:  Through the
  use of a packet queue and an alert pipe, the TCP helper thread
  can now be woken up to write data as well as read data.
3.Locking error: sip_xmit returned an XMIT_ERROR without giving
  up the tcptls_session lock.  This lock has been completely removed
  from sip_xmit and placed in the new sip_tcptls_write() function.
4.Memory leak:  When creating a tcptls_client the tls_cfg was alloced
  but never freed unless the tcptls_session failed to start.  Now the
  session_args for a sip client are an ao2 object which frees the
  tls_cfg on destruction.
5.Pointer to stack variable: During sip_prepare_socket the creation
  of a client's ast_tcptls_session_args was done on the stack and
  stored as a pointer in the newly created tcptls_session.  Depending
  on the events that followed, there was a slight possibility that
  pointer could have been accessed after the stack returned.  Given
  the new changes, it is always accessed after the stack returns
  which is why I found it.

Notable code changes
1.I broke tcptls.c's ast_tcptls_client_start() function into two
  functions.  One for creating and allocating the new tcptls_session,
  and a separate one for starting and handling the new connection.
  This allowed me to create the tcptls_session, launch the helper
  thread, and then establish the connection within the helper thread.
2.Writes to a tcptls_session are now done within the helper thread.
  This is done by using an alert pipe to wake up the thread if new
  data needs to be sent.  The thread's sip_threadinfo object contains
  the alert pipe as well as the packet queue.
3.Since the threadinfo object contains the alert pipe, it must now be
  accessed outside of the helper thread for every write (queuing of a
  packet).  For easy lookup, I moved the threadinfo objects from a
  linked list to an ao2_container.

(closes issue #13136)
Reported by: pabelanger
Tested by: dvossel, whys

(closes issue #15894)
Reported by: dvossel
Tested by: dvossel

Review: https://reviewboard.asterisk.org/r/380/



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@225445 65c4cc65-6c06-0410-ace0-fbb531ad65f3