[0.89-fb] [HBASE-8228] Enable finer profiling for a memstore snapshot. -- TEMPORARY DIFF

Author: aaiyer

Summary:
more instrumentation

We do not intend to have this diff around for a while. Will be
reverted after getting the info we need to track the outliers.

Test Plan: push to titan025 and get the data to understand this.

Reviewers: liyintang, rshroff, adela

Reviewed By: rshroff

CC: hbase-eng@

Differential Revision: https://phabricator.fb.com/D758003

Task ID: 2237950

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1463658 13f79535-47bb-0310-9956-ffa450edef68