XD-1596 Expose Common Rabbit Source/Sink Options

JIRA: https://jira.spring.io/browse/XD-1596

Source:

Declare the container as a `<bean/>` and expose common
configuration options (concurrency etc), including
`maxConcurrency` which is not currently available on the
Spring Integration namespace.

Sink:

Add delivery mode.

Both:

Allow configuration of the message converter by class
name (e.g. Simple Vs. JSON).

Exercised by `RabbitSourceAndRabbitSinkTests`