[HBASE-8307] Improving the Histogram class to work fine in underload case

Author: manukranthk

Summary: Add a list to store and serve the percentile metrics when the load under a default load. This will help in avoiding the roundoff errors in the cases with low samples.

Test Plan: Unit test. Test on Shadow.

Reviewers: liyintang, rshroff

Reviewed By: liyintang

CC: hbase-eng@

Differential Revision: https://phabricator.fb.com/D802945

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1480733 13f79535-47bb-0310-9956-ffa450edef68
[HBASE-8512] Make getCurrentPoolSize public

Author: shaneh

Summary: Makes the HTablePool function public.

Test Plan: Tested on my dev server.

Reviewers: liyintang, jungejason

Reviewed By: jungejason

CC: hbase-eng@

Differential Revision: https://phabricator.fb.com/D804891

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1480734 13f79535-47bb-0310-9956-ffa450edef68