INT-3325 Add Redis Channel Message Group Store

JIRA: https://jira.spring.io/browse/INT-3325
JIRA: https://jira.spring.io/browse/INT-1870

Optimized MGS for QueueChannel - uses a LIST for
each channel and LPUSH, RPOP.

* Also fix MutableMessage to be Serializable

INT-1870 Priority Redis Channel Message Store

Supports priorities 0-9 (+ no priority).

Priorities out of that range are treated as no priority.

Polishing - Add Marker Interfaces

* Emit a `WARN` log if a channel is used with a regular MessageGroupStore
* Allow message-store on namespace when defining a priority channel

INT-3325 Polishing; PR Comments

Fix some typos in JavaDocs and Docs
INT-3328: Add `EnablePublisher` Annotation

JIRA: https://jira.spring.io/browse/INT-3328

INT-3328: Polishing
INT-3332 GlobalChannelInterceptorBeanPostProcessor

Implicit channel declaration for items downstream
of a `ChannelInterceptor` were not created.

The `BPP` eagerly fetched the interceptors during its
own creation; this caused the context initialization to fail
because the channel initializer hasn't run yet.

Defer creation of the interceptor beans until they are
actually needed.

Also, when using `@Configuration`, the channelInitializer
is no longer the first bean in the bean factory.

INT-3332 Use SmartLifeCycle to Apply Interceptors

Instead of using a bean post processor, the interceptor
processor now performs the channel interception when beans
in phase Integer.MIN_VALUE are started - after all beans
have been instantiated.

Polishing
INT-3336 Add error-channel to MQTT m-d-c-a

JIRA: https://jira.spring.io/browse/INT-3336

Previously exceptions thrown in a flow downstream of a
message-driven-channel-adapter were not logged and thrown
back to the client, causing the connection to drop and
reconnect.

Add `error-channel` to the adapter to allow normal
error handling. If no error channel, catch and log
the unhandled exception.

Add `adapter.stop();` in the end of 'real' tests to close the mqtt-connection
INT-3336 Fix MQTT Test Rule

JIRA: https://jira.spring.io/browse/INT-3336

Disconnect the client after verifying the broker is available.
Change the rule to a class level rule in DownstreamExceptionTests.