Resolves: /spring-projects/spring-integration#2268
* To indicate the source the Redis message in the `RedisInboundChannelAdapter`
populate the `RedisHeaders.MESSAGE_SOURCE` header to the messages to produce
* Fix the `SimpleMessageConverter` to populate the provided `MessageHeaders`
to the message to produce
* Add `toMessage(T object, @Nullable Map<String, Object> headers)`
to the `InboundMessageMapper` to propagate additional header
to the message to create
* Rework all the out-of-the-box `InboundMessageMapper` implementations
to properly propagate additional headers via `toMessage()`
from the `SimpleMessageConverter`
* Provide optimizations in the `InboundMessageMapper` implementations
do not re-create messages
* Refactor `MutableMessage.toString()` to align with the `GenericMessage`
* Add more `@Nullable` to method arguments
* Increase latch wait timeout in the `EndpointParserTests`
* Address `redis.adoc` PR comment
7ca20e53f7