Added the LOCK and UNLOCK commands. LOCK will lock the file mutex and

Added the LOCK and UNLOCK commands. LOCK will lock the file mutex and
prevent other clients with the same open file from sending non-cache
related commands until UNLOCK is sent from the same client that sent
LOCK.

So if you are in the middle of modifying some existing data and you
have a client that needs the same data, this can prevent old data
being sent to the other client and can also prevent
EPWMD_FILE_MODIFIED errors.