HBASE-8787 Merged region endkey is incorrect if one region has null endkey

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1495745 13f79535-47bb-0310-9956-ffa450edef68
HBASE-8750 MetaServerShutdownHandler stucks if .META. assignment fails in previous attempt

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1495748 13f79535-47bb-0310-9956-ffa450edef68