BlockMgr reservation improvements.

Change-Id: I9c7fa0ce54cbbf5b2a7a368c27c379c2a5241fc8
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4732
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5092
Some test case cleanup.

Change-Id: Ic29b7c1f5fd714a1e2cc41bf0e55c0d11c782862
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4791
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: jenkins
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5090
Reviewed-by: Nong Li <nong@cloudera.com>