Protect "unknown_command" to introduce datatype

The client need to call HELLO in order to be allowed
to use the datatype field of the header. This patch returns
EINVAL if the client suddenly decide to use it anyway for one
of the engine-specific commands.

Change-Id: I1534e5d24cff69824abd006b0daa742bb3db83ff
Reviewed-on: http://review.couchbase.org/38226
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>