[0.89-fb] [HBASE-8476]  locateRegionInMeta should check the cache before doing the prefetch

Author: aaiyer

Summary:
When we start a hbase client, all threads
need to synchronize to avoid accessing the META region
at the same time.

However, we need to check if the region cache is populated
before going to META. Without this, each thread will end up
fetching entries from META, losing any possible optimisation
for reducing the queries to META.

Test Plan:
pushed to titanshadow025 and see  a much faster
htable opens.

 run mr unit tests

Reviewers: rshroff, manukranthk, liyintang, anshumansingh26

Reviewed By: anshumansingh26

CC: hbase-eng@, anshumansingh26

Differential Revision: https://phabricator.fb.com/D795230

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1478134 13f79535-47bb-0310-9956-ffa450edef68