HIVE-6518: Add a GC canary to the VectorGroupByOperator to flush whenever a GC is triggered. (Gopal V via jitendra, reviewed by Remus, Gunther)

git-svn-id: https://svn.apache.org/repos/asf/hive/branches/branch-0.13@1578501 13f79535-47bb-0310-9956-ffa450edef68