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

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.95@1510800 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/0.95' into 0.95-gd
Merge remote branch 'origin/0.95-gd' into fb-YAHOO-323-0.95-gd