HBASE-9095. AssignmentManager's handleRegion should respect the single threaded nature of the processing

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1510799 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk-gd
Merge remote branch 'origin/trunk-gd' into fb-HBASE-8559-trunk-gd