[FTS] improve rebalance behavior

FTS currently takes a highly conservative approach to rebalance, moving just a single shard at a time.  This combines in a negative way with two general observations about rebalance:

• too slow
• don't know what is currently doing

Three main tasks have been identified to possibly improve this in time for the next release.

1) Documentation around the new impacts to rebalance by FTS and other MDS services

2) Make an attempt to tune the default values for FTS rebalance so that the out of the box behavior is reasonable.

3) Consider adding an FTS endpoint which might expose finer-grained details about the FTS rebalance progress.  The UI may have no way to consume or present this information, but it could be used by the support team to aid in helping customers understand the situation.