BUG-8403: fix DONE state propagation

BUG-8403: fix DONE state propagation

The DONE state detection logic in replayMessages() was flawed, as
we checked the current state, which is guaranteed to be SuccessorState.

We should be checking the previous state, available from the successor
state. As it turns out we can do this very cleanly by setting the flag
when the successor state gets the previous state assigned.

This also has better performance, as we do not touch the volatile
field multiple times.

Change-Id: Ica2246160bf8fee7aa134bbacb45857235405f6a
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>