BUG-8618: refresh transaction access when isolated

When we are isolated leader we stop accepting messages from
the frontend. If we remain in this state for more than 15 seconds
this can result in a timeout -- which is obvious, but it really
is our fault.

Since we cannot make forward progress anyway, there is no point
in purging the transaction. Update its access time with whatever
the last mark for that frontend was.

Change-Id: I9ff56c91e4fda4b68cd34c05609dc88d6d65fd32
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>