INT-3455 Orderly Shutdown Improvements

JIRA: https://jira.spring.io/browse/INT-3455

Do not stop schedulers and executors - allows mid-flow
QueueChannels to be drained.

Stop all inbound MessageProducers (that are not OrderlyShutdownCapable).
Already did it last night :smile: https://github.com/spring-projects/spring-integration/wiki/Spring-Integration-4.0-to-4.1-Migration-Guide

(Looks like Liu didn't start a new section for his change).