INT-3560: Fix DelayHandler for Duplicate Messages

JIRA: https://jira.spring.io/browse/INT-3560

Previously, messages arriving at the delayer before the
the context was initialized would be emitted twice after
the context `refresh()` or a JMX invocation of `reschedulePersistedMessages()`.

When using a `SimpleMessageStore`, the "re" scheduled
message from the context refreshed event (or a JMX invocation)
would be re-handled unconditionally.

This was due to incorrect logic to handle the way the `SMS` stores messages.

Change the logic to correctly handle (ignore)  duplicate scheduled
releases when using `SMS`.

Conflicts:
	spring-integration-core/src/main/java/org/springframework/integration/handler/DelayHandler.java
	spring-integration-core/src/test/java/org/springframework/integration/handler/DelayHandlerTests.java