INT-2634: Test for gateway's `Map` parameter

JIRA: https://jira.spring.io/browse/INT-2634

The Map entries with non-String keys are now skipped just with WARN message,
instead of `IllegalArgumentException` as it was before
INT-3200: SFTP: use `byte[]` for the PrivateKey

JIRA: https://jira.spring.io/browse/INT-3200

INT-3200: Remove reduntant test case

INT-3200: remove trailing space
Update CONTRIBUTING.md
Fix Occasional Reaper Expiry Test Failure

https://build.spring.io/browse/INT-MJATS41-32/

Also add a test for expiring a group within an aggregator.

Polishing
INT-3402: Late resolution for channel names

JIRA: https://jira.spring.io/browse/INT-3402

* Move `channel name to channel` resolution logic to the action, e.g. `sendReplyMessage`
* Remove resolution logic from `MessagingAnnotation` Processors
* Fix `AggregatorAnnotationPostProcessor` to invoke `handler.afterPropertiesSet()` one more time,
because of `AbstractMethodAnnotationPostProcessor` `this.beanFactory.initializeBean(handler, handlerBeanName)`
* `Filter` annotation: allow `property-placeholder` for the `discardWithinAdvice`
* Fix `EnricherParserTests4-context.xml` duration: use just `0` for the `reply-timeout`
INT-3402: Add `synchronized double check`
INT-3402: Polishing according PR comments
INT-3402: Optimization for `AbstractCorrelatingMH`