FavoredNodes - Split/Merge code paths

This jira is to deal with the split/merge logic discussed as part of HBASE-15532. The design document can be seen at HBASE-15531. The specific changes are:

Split and merged regions should inherit favored node information from parent regions. For splits also include some randomness so even if there are subsequent splits, the regions will be more or less distributed. For split, we include 2 FN from the parent and generate one random node.