Only update rebalance time if nodes are rebalanced.

This commit changes TxStats.RebalanceTime to only update if there are nodes that have been
rebalanced. Previously, the RebalanceTime would include a small amount of time to check if
there were nodes to rebalance but this is confusing to users and not terribly helpful.
Merge pull request #224 from benbjohnson/rebalance-time

Only update rebalance time if nodes are rebalanced