This was found by the first assertion of answerRebaseObject (client) because
a storage node missed a few transactions and reported a conflict with an older
serial than the one being stored: this must never happen and this commit adds a
more generic assertion on the storage side.

The above case is when the "first phase" of replication of a partition
(all history up to the tid before unfinished transactions) ended after
that the unfinished transactions are finished: this was a corruption bug,
where UP_TO_DATE cells could miss data.

Otherwise, if the "first phase" ended before, then the partition remained stuck
in OUT_OF_DATE state. Restarting the storage node was enough to recover.