[master] Fix race condition when opening region on stopping region server

Author: fan

Summary: The region server aborts opening region when it detects a shutdown.

Test Plan:
Added a new unit test to simulate this scenario.
When I disable the fix, the unit test detects a leftover online region.

Reviewers: liyintang, aaiyer, mycnyc, jiqingt

Reviewed By: jiqingt

CC: HBase Eng@, hbase-prodeng@

Differential Revision: https://phabricator.fb.com/D1120497

Task ID: 3496863

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1556924 13f79535-47bb-0310-9956-ffa450edef68