INT-2878: MessageGroupStoreReaper's Lifecycle fix

`MessageGroupStoreReaper` continues to expire `MessageGroups` via `scheduled-task` independently of its `running` state.

* Add check `isRunning()` to `run()` & `destroy()`
* Additional `MessageGroupStoreReaper` polishing
* Add test-case

JIRA: https://jira.springsource.org/browse/INT-2878