[HBASE-6728] [89-fb] Improvements to SizeBasedThrottler

Author: adela

Summary:
- current implementation had all methods syncrhonized - which means
  synchronization overhead was always incurred - and in contrary to
  comments in code. Removing all synchronized modifiers from methods
  actually does what comments explain (and was probably initially
  intended)
- further - removed actual race condition (which happens only after we
  remove syncrhonized modifiers)

Test Plan: run TestSizeBasedThrottler

Reviewers: aaiyer, liyintang

Reviewed By: liyintang

CC: manukranthk, hbase-eng@, rshroff, shaneh

Differential Revision: https://phabricator.fb.com/D778193

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1489543 13f79535-47bb-0310-9956-ffa450edef68