Allow session storage to be configured independently

Allow session storage to be configured independently

* Introduce $wgSessionCacheType to allow the session storage to be
  configured independently of $wgMemc.
* Renamed $wgSessionsInMemcached to $wgSessionsInObjectCache to reflect
  the new versatility.
* Modernized the relevant code, removing the explicit require_once(),
  used the autoloader instead. Moved it to ObjectCacheSessionHandler.
* Tested with memcached, external sharded MySQL and APC, it all seems to
  work just fine.

Change-Id: I473334bb56cafb4e21ac1c1304d69095676fc0c4