Disconnect clients sending unknown commands

Change-Id: Ifc9af146ae1a53da2c3ee694a4edef0df45fa6d7
Reviewed-on: http://review.couchbase.org/28874
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>