Make FindAllTabletAliasesInShard better.

It now uses the new replication graph.
Fix InitTablet to write master to shard.
Re-working ShardExternallyReparented.

So it works with Shard having the master record.
Moving a function.
Merge branch 'master' into replication
Not changing a shard master during rebuilds now.
Make external reparent more robust.
Merge branch 'master' into replication
Adding comment.
Now Reparenting actions use new replication graph.
Removing a few obsolete methods and code.
Merge branch 'master' into replication
Merge branch 'master' into replication

Conflicts:
	go/vt/wrangler/reparent_external.go
Handle cell-level failures in reparent_external.
Merge branch 'master' into replication
Merge branch 'master' into replication