Augment internal queue interface. Update BufferedBlockMgr to use it more.

Change-Id: I662fde6165726767787b722f5b74d10f94fe158c
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3543
Reviewed-by: Marcel Kornacker <marcel@cloudera.com>
Tested-by: jenkins
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3585
Reviewed-by: Nong Li <nong@cloudera.com>