INT-3564 Update Javadoc URLs to spring.io

JIRA: https://jira.spring.io/browse/INT-3564

INT-3564 Update Javadoc Stylesheet for JDK8
INT-3571: Propagate Lifecycle from SPCA to MS

JIRA: https://jira.spring.io/browse/INT-3571

**Cherry-pick to 4.0.x**

Introduce `LifecycleMessageSource` for `maint` versions to avoid undesired side-effects with already existing `... implements MessageSource<T>, Lifecycle`.
INT-3568: Improve `Endpoint Summary`

JIRA: https://jira.spring.io/browse/INT-3568
INT-3563: Add router's `id` to the MDException

JIRA: https://jira.spring.io/browse/INT-3563
INT-3572: Improve Accept Once Filter Performance

JIRA: https://jira.spring.io/browse/INT-3572

Previously, a blocking queue was used to hold seen files; this was used to enable
FIFO when a max capacity is set.

When used with no capacity, a queue is not needed; also the `contains` operation
on a queue requires a linear search which does not scale well for a large number
of files.

Use a `HashSet` instead to significantly improve the `contains` performance.

Only maintain a queue if a max capacity is set.

Fix `AcceptOnceFileListFilterTests` to be compatible with Java < 8
INT-3573: Fix `EEvalSplitter` for `Iterator`

JIRA: https://jira.spring.io/browse/INT-3573

Fix `seen` `Queue` `NPE` for `(S)FtpInboundRemoteFileSystemSynchronizerTests`:
https://build.spring.io/browse/INT-B41-JOB1-164/test/case/155357717

**Cherry-pick to 4.0.x**
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`.
INT-3576: Fix NPE in the `RedisAvailableTests`

JIRA: https://jira.spring.io/browse/INT-3576
INT-3575 TcpNetConnection: flush stream in send()

JIRA: https://jira.spring.io/browse/INT-3575