Rewind on uuid or epoch change

Only use the StartSeq for a shard if we are sure it's the original
shard.

Moving a shard will add a new epoch, starting at the first committed
update sequence since the move. Originality is defined as;

* uuid's must match
* the node of the ultimate epoch must match the since node
* the seq of the penultimate epoch, if there is one, must be lower
  than the since seq.

If any of these tests fail, the changes feed for that shard rewinds to 0.

BugzID: 18726