Online config change

Summary: This diff is intended to forward port HBASE-8805 and HBASE-8544 implemented by Gaurav Menghani in 89-fb. This improves operational efficiency in managing clusters that are serving production traffic. The idea is to have a central configuration which can manage notifying the configuration observers. The observers in turn should update their local state from the latest config. Minor caveats where configuration variables are corelated should be taken care of with additional care.

Test Plan: Unit tests

Differential Revision: https://reviews.facebook.net/D24681

Signed-off-by: stack <stack@apache.org>
HBASE-12147 Porting Online Config Change from 89-fb -- JAVADOC FIX ADDENDUM