IMPALA-911: Periodically release memory in tcmalloc.

Change-Id: Ifc3d544b7bdc4caa2f238ae13cab6a62cfc587f9
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2066
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins