Revert "LU-2675 obd: decruft md_enqueue() and md_intent_lock()"

This reintroduced back LU-4805, so reverting.

This reverts commit a2877f1f6c0fd0ec5ece836a216f42af3ef913da.

Change-Id: Id6728d4fa6a322a45579f0ce744d4c7161ac9d69
Reviewed-on: http://review.whamcloud.com/10203
Tested-by: Jenkins
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Tested-by: Oleg Drokin <oleg.drokin@intel.com>