CasMismatch is not thrown for Unlock with an incorrect CAS

Likely related to NCBC-3681.

Performing an Unlock() on a document and providing a wrong CAS value for it is supposed to throw a CAS mismatch exception.

There is also a note in the RFC:

• Note: do NOT retry on KV_LOCKED for the "Unlock" command (the retry handler should never see it in the first place!( Reason is that this needs to be surfaced as a cas mismatch exception to the user and not being retried