Add Scatter-Gather pattern, and make it easy to configure

Scatter-Gather is quite a common and useful pattern. You can implement it in Spring Integration but it's long-winded and wasteful of channel schedules, e.g.

<integration:handler-endpoint input-channel="requests" handler="splitter"/>
<integration:handler-endpoint input-channel="splits" handler="router" />
<integration:handler-endpoint input-channel="scatters" handler="aggregator" />
<integration:router id="router" ref="scatterer" method="route" />
<integration:splitter id="splitter" ref="scatterer" method="split" output-channel="splits" />
<integration:aggregator id="aggregator" ref="scatterer" default-reply-channel="replies" method="aggregate" />

This example uses a single POJO (called "scatterer") to synthesise a scatter and gather. It first splits the incoming message on "requests" and then routes the resulting messages to a pre-specified set of channels (names injected into the POJO). Then it also acts as an aggregator, waiting for the requests to come back in to the "scatters" channel.