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.
In case we want to add some other scheme in the future.

Current:

    <len><msg><len><msg>

Maybe in future:

    <directory><msg><msg><msg>

Also, if a user provides his own batching strategy, we wouldn't know how to decode it so we pass the un-decoded message to the listener.