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-3519 Add Poll Skip Advice

JIRA: https://jira.spring.io/browse/INT-3519

Add a mechanism to allow skipping polls, perhaps
because of some downstream condition.

INT-3519: Polishing