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.
So, `length == 4` is for 4 bytes of integer?