HBASE-12139 StochasticLoadBalancer doesn't work on large lightly loaded clusters

Summary:
Currently the move cost overshadows the skew cost on a large cluster. This can render the split policy worse than useless
and it can trap meta on the most loaded server in the cluster.

Test Plan:
Unit tests everywhere
likeaboss

Reviewers: stack, manukranthk

Subscribers: tedyu

Differential Revision: https://reviews.facebook.net/D24285