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-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
INT-275: Fix `ScatterGatherHandler` JMX proxying issues

* Make `AbstractCorrelatingMessageHandler.getMessageStore()` as `public`
* Move `ScatterGatherHandlerIntegrationTests` to the JMX module to be sure that `ScatterGatherHandler`
works well with `@EnableIntegrationMBeanExport`
* Add xml config sample how to use an internal gatherer's `MessageStore` in the `MessageGroupStoreReaper`
* Add `What's New` notice