INT-275: Implement Scatter-Gather pattern

JIRA: https://jira.spring.io/browse/INT-275

In addition fix the `Lifecycle` issue in the `ServiceActivatorAnnotationPostProcessor`
I think we need to expose a getter for the gatherer's message group store. With the default configuration, completed groups (empty) will be retained in the MGS and we'll eventually OOM.

    <bean class="MGS">
        <constructor-arg value="#{sg.handler.messageGroupStore}" />
    </bean>