Fixes: https://github.com/spring-cloud/spring-cloud-stream/issues/2939
The RabbitMQ 4.0 does not deal with client side `x-*` headers.
Therefore, an `x-death.count` is not incremented anymore when
message is re-published from client back to the broker.
* Spring AMQP 3.2 has introduced an `AmqpHeaders.RETRY_COUNT` custom header.
Use `messageProperties.incrementRetryCount()` in the `RabbitMessageChannelBinder`
when we re-published message back to the broker for server-side retries
* Fix docs respectively
Resolves#3019
