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
8ac4bfbd2c