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