Adding get_lock and release_lock support

Currently the features below are not supported when we use maxscale and a router that can end up on multiple servers.
GET_LOCK()
RELEASE_LOCK()
IS_FREE_LOCK()
IS_USED_LOCK()

If we do a get_lock and get routed to serverA and later on we do a release_lock and this is routed to serverB the lock on serverA is never released.
It can be used if force the router to always go to the master server.

Because of this we can not use the readwrite split router.