INT-3477: Add Reactor Promise<?> for Gateway

JIRA: https://jira.spring.io/browse/INT-3477
That's true and it is a right behaviour: see `EnableIntegrationTests#testPromiseGateway()` and docs.
You can simply `flush()` the `Promise<?>` just after return via method chain, if you really are interested in just a result of `Promise<?>`, like for `Future<?>`. 
But from other side `Promise<?>` is more flaxible and it can be a part of another Reactor `Stream<?>`, when you really want `flush` an entire stream.
It is a hook how to get deal with SI from Reactor - `poll` aspect.

We talked a bit with @smaldini that another hook should `push` from SI to Reactor. Something like this:
![reactorworker](https://cloud.githubusercontent.com/assets/1071590/3807899/27562d8e-1c6f-11e4-9ffe-c4dd7789943c.jpg)
Where SI message will send to specific `Reactor` by `selector`.