Fix SrvShard structure.

No more VtnsAddrs nor ReadOnly (was not used and not updated),
added a TabletTypes, as it is used to compute the keyspace's
TabletTypes.
Fixing RebuildReplicationGraph.

It was broken, but there was no test. Adding a test for it.
Also hiding the command from the 'vtctl -h' list of commands,
as it is dangerous.