Reconsider the rebalance early exit check

The rebalance early exit check was added to avoid redistributing partitions in an already balanced cluster, given that each partition move resulted in file transfer, setting up feeds, etc. 

An alternative to the check would be to ensure that the inputs to the plan computation API for each index's rebalance are idempotent if there is no change in partition count, node layout, etc. The plans computed for the index would be identical to the plans computed prior, avoiding the need for an early exit without partition movement.