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-275: Implement Scatter-Gather pattern

JIRA: https://jira.spring.io/browse/INT-275

In addition fix the `Lifecycle` issue in the `ServiceActivatorAnnotationPostProcessor`
Add Namespace support, Addition tests
   and fix some typos in the XSD
INT-275: Fix failed tests
INT-275: Fix for `replyChannel` Header

Add `sync` reply test-case
INT-275: Make `ScatterGatherHandler` sync

Fix some `MessageHandler`s from `SmartLifecycle`
INT-275: Fix `ScatterGatherHandler.handleRequestMessage` logic