INT-275: Implement Scatter-Gather pattern

JIRA: https://jira.spring.io/browse/INT-275

In addition fix the `Lifecycle` issue in the `ServiceActivatorAnnotationPostProcessor`
OK. I'll redo that to an internal Gateway with an internal error-channel.
Having gateway we can receive gather result from Aggregator independently of Aggregator's options, or `forceComplete` the last one in case of receive-timeout.

>  how do we wake the caller in that case?

In case of Gateway we just receive `reply` from reply-channel. The gateway does the stuff.

As you see my current solution doesn't care about calling thread and just shift gatherer message to the output channel through a `FixedSubscriberChannel`. And it really doesn't matter how an aggregator produces its result.

Re. `RLR`. Even if EIP site doesn't say anything on the matter (http://www.enterpriseintegrationpatterns.com/BroadcastAggregate.html), please open EIP Book. There is really a note about `Distribution`. And see my test-case: https://github.com/spring-projects/spring-integration/pull/1281/files#diff-75fdc122f9334de56379af666c059b2bR12