INT-2878: MessageGroupStoreReaper's Lifecycle Fix

`MessageGroupStoreReaper` continues to expire `MessageGroups` via `scheduled-task`
when not `running`.

* Add check `isRunning()` to `run()` & `destroy()`
* Additional `MessageGroupStoreReaper` polishing
* Add test-case

JIRA: https://jira.springsource.org/browse/INT-2878

INT-2878: Polishing : PR Comments
Merge pull request #704 from artembilan/INT-2878