AMQP-451 Batching RabbitTemplate

JIRA: https://jira.spring.io/browse/AMQP-451

__Released in stealth mode for now (no docs, javadocs marked `Experimental`).__

This feature is to support XD; we can't incubate in XD because it needs a
change to the listener container.
The test case needs to verify that the bad message was rejected; it does that by spying the logger. You can't spy (update) a final static field.

Also, `protected static...` implies it is available for subclasses so a hard-coded classname is not right anyway :smile: