Reparent and rebuild changes.

Two fixes to improve the stability of the system:

- when we ShardExternallyReparent, if a slave has the wrong master,
we keep it in the replication graph with its current master.
That way it will still be discovered properly.

- when we rebuild the serving graph, we skip tablets which have
the wrong master (as in: not the master in ShardInfo). They're not
replicating properly, they shouldn't be serving.