BUG-8620: handle direct commit and disconnect correctly

BUG-8620: handle direct commit and disconnect correctly

Transactions committed directly can complete in a disconnected
fashion as we are skipping the back-and-forth communication of the
three-phase commit. This period may involve shard leadership changes
and so we may end up in a situation where we are replaying a direct
commit request to a transaction which already completed -- which
raises a RequestFailure to make sure we do not do anything untoward.

In the specific case of direct commit, though, this is perfectly fine
and so update the callback to account for this case happening.

Change-Id: Ic60e69f0f58cc7c5a3ac869386dc12f856aa1f74
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>