Changes from 22/9/2014.

Changelog:
2014.09.22:
- Changed the way in which the "RTSPServer" code handles incoming "OPTIONS" commands that contain
  a "Session:" header.  If the "Session:" header contains a session id that does not exist, then
  we now return a "Session Not Found" error (even though the handling of the "OPTIONS" command is
  not session-specific).  This new behavior will help proxy servers (that use our "RTSPServer"
  implementation as a 'back-end' server) better detect when the back-end server has restarted
  while streaming.  (Thanks to Jonathan Brady for noticing this issue.)