Fixes#5680.
When dropping a data node, the following will now happen on the
Meta Store.
1) If any shards no longer have any owners (because the data node
being dropped is the only owner), they will be reassigned a
new owner from within their respective shard group.
2) If a shard group no longer has any shards/data nodes, they will
be marked as deleted.
When a shard is being assigned a new owner a data node with the fewest
number of shards in the shard group will be selected as the new owner.
Finally, checking the validity of a data node's ID now happens in the
Meta store, rather than in the state machine.
Cory LaNou
Edd Robinson