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

JIRA: https://jira.spring.io/browse/INT-3477

INT-3477: Fix `reactorEnvironment` propagation

* `MessagingGatewayRegistrar` parser the value for the `reactorEnvironment`
* Provide more interest test-case
* Polishing docs

INT-3477 Require `Environment` in case of Promise

* Do not use Reactor `Environment` as default instance.
* Require `Environment` reference, when is `Promise` method
* Polishing for tests
* Apply Gary's polishing for docs

INT-3477: Assert.notNull -> Assert.state

INT-3477: PR review