HBASE-8945 Backport to 0.94: HBASE-7952 Remove update() and Improve ExplicitColumnTracker performance. (orginal path by Raymond Liu)

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.94@1504313 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/0.94' into 0.94-gd
Merge remote branch 'origin/0.94-gd' into fb-YAHOO-316-0.94-gd