INT-3516: Allow `Optional<>` in POJO Method Args

JIRA: https://jira.spring.io/browse/INT-3516

* Add `Optional<>` support for `@Header` in the `MessagingMethodInvokerHelper`
* Add `Optional<>` test-case
* Change `sourceCompatibility` for test to the Java 8

INT-3516: Revert SF version to 4.1.1

Add Docs on the matter

Fix WS Tests; Revert StubJavaMailSender
INT-3520: Add Reactor's `PersistentQueue` Support

JIRA: https://jira.spring.io/browse/INT-3520

INT-3520: Use `Condition` to wait items in the `Queue`

INT-3520: Reworking to the `Semaphore(0)`

* Implement 'artificial infinite wait'
* Add distributed test with infinite `receive()`

INT-3520: Correct usage of `Semaphore`

Minor Doc Polishing
INT-3516 Fix JPA Build Error

https://build.spring.io/browse/INT-B41-121
INT-267: Implement RoutingSlip pattern

JIRA: https://jira.spring.io/browse/INT-267

The implementation looks like:
* There is the `ROUTING_SLIP` header to keep the list of bean ids;
* The `ROUTING_SLIP_INDEX` header keeps track of current `index` in the `ROUTING_SLIP` header;
* `ROUTING_SLIP` List can contain channel names or bean references for the `RoutingSlip` strategy implementations.
They are differentiated with `@` prefix;
* The `<header-enricher>` adds `<routing-slip>` sub-element to specify the comma-delimited value for desired `ROUTING_SLIP` for the downstream flow;
* The `AbstractReplyProducingMessageHandler` adds the logic to get deal with `ROUTING_SLIP` List and the algorithm is:
    - If `ROUTING_SLIP` isn't `null` we build `AtomicInteger` for the current `routingSlipIndex`;
    - the recursive `getReplyChannelFromRoutingSlip` should return a channel name or `null`;
    - if current `ROUTING_SLIP_INDEX` if for the `RoutingSlip` strategy, we check its result for `null` and `incrementAndGet()` the current index or not;
    - for the simple channel name value from `ROUTING_SLIP` list we just `incrementAndGet()` the current index and return the value;
    - the new `ROUTING_SLIP_INDEX` is populated to the headers of new reply message.
* Polishing for `AbstractMessageSplitter`

**TODO**: Docs and applying `RoutingSlip` algorithm for the `AbstractCorrelatingMessageHandler`
INT-267: Move `replyProducing` logic `ARPMH` -> `AMPH`

* Rework `AbstractCorrelatingMessageHandler` to use methods from super class
* Rework `MessageHandlerChain.ReplyForwardingMessageChannel` to use `produceReply`
* Rename `RoutingSlip` -> `RoutingSlipRouteStrategy`
* Add `ExpressionEvaluationRoutingSlipRouteStrategy`

Now `routingSlip` header can be configured like:

```
<routing-slip value="channel1; #{@routingSlipRoutingPojo.get(request, reply)}; @routingSlipRoutingStrategy; #{request.headers[myRoutingSlipChannel]}; channel6"/>
```

Where `;` is used as delimiter, because of `,` in the method invocation from SpEL.
The simple literal (`channel1`) is just a `MessageChannel` `id`.
`@` is used for `RoutingSlipRouteStrategy` bean reference.
`#{...}` used for SpEL.
The `HeaderEnricherParserSupport` parses this `value` to the `List<String>` - a set of bean names, where any SpEL is wrapped
to the `ExpressionEvaluationRoutingSlipRouteStrategy` bean definition
INT-267: Introduce `RoutingSlip` Domain class

Rename `AbstractMessageProducingHandler` methods: `*reply` -> `*output`
INT-267: Rework `RoutingSlip` -> `Map<List<String>, Integer>`

Since `RoutingSlip` POJO isn't scalable in the distributed multi-language environment,
it would be better to use some Java generic type for this `ROUTING_SLIP` header.

The `Collections.singletonMap(Collections.unmodifiableList(routingSlipPath), 0)` is the best candidate to be convertible to other systems
and allow to have thread-safety.

The `ROUTING_SLIP` header is recalculated now on each `nextPath`
INT-267: Introduce `RoutingSlipHeaderValueMessageProcessor`

* Rework `HeaderEnricherParserSupport` logic to get rid of SpEL parsing
and change `routingSlipPath` to the `ManagedList<String>` to get gain of `property-placehoder`
* Add `<context:property-placeholder>` stuff to the `RoutingSlipTests`