JIRA: https://jira.spring.io/browse/INT-4012
Previously to configure a `priority` for the `MessageChannel` we should configure `QueueChannel` for particular `MessageStore`.
Although the target priority logic is really in the `MessageStore` implementation, it isn't so obvious why we can't use `PriorityChannel` instance for `MessageStore` case as well.
* Add `PriorityCapableChannelMessageStore` and `MessageGroupQueue` based ctors to the `PriorityChannel` for consistency.
* Delegate the logic to the super `QueueChannel` as before
* Rework `PointToPointChannelParser` and Java DSL components to reflect a new state of the `PriorityChannel`
* Improve Docs on the matter
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