INT-3338: MongoMS: Add `priority` and `sequence`

JIRA: https://jira.spring.io/browse/INT-3338

INT-3338: Add Docs

INT-3338: Make `priority` 'smart'

Add `Sort` for all group queries dependently of `priorityEnabled`.
Avoids the need to configure separate collections for different type of `MessageStore`

INT-3338: Add `MongoDbChannelMessageStore`

INT-3338: Polishing, Fixes, Improvements

Doc Polishing

Fix stream test.