Rebalance range replica. Only fully-replicated ranges may be rebalanced.

[petermattis]

Keep a rebalance queue in memory. Range replicas are added to the queue from a store during range scan according to certain conditions:

• Replica not matching zone config. When zone config changes happen, all ranges are scanned by each store and any mismatched replicas are added to the queue.
• Store is overweight in terms of size or range count.
• Rebalance away from stores finding themselves in top N space utilized, taking care to account for fewer than N stores in cluster. Only stores finding themselves in the top N space utilized set may have rebalances in effect.

[spencerkimball]

@mrtracy, would you care to create a follow on issue, or issues, for what else needs to be done for beta. Perhaps improvements to the heuristics, as necessary.