INT-1870: Add MessageStore to PriorityChannel

Previously, `<priority-channel>` didn't support `message-store`
and the priority logic was based only on `PriorityBlockingQueue` and `SequenceFallbackComparator`.

* Introduce `PriorityMessageGroupQueue`
* Add `MessageGroupStore#pollMessageFromGroupByPriority`
and implement it for all `MessageGroupStore`s
* Make `<priority-channel>` `comparator` and `message-store` attributes mutually exclusive
* Add `PriorityChannel` tests for all `MessageGroupStore`s

JIRA: https://jira.springsource.org/browse/INT-1870