INT-3426: Add `<r-h-a-c>` for all `outbound-c-a`

JIRA: https://jira.spring.io/browse/INT-3426

Provide other fixes for XSD. E.g. not all adapter have had `<poller>`
INT-3415 Add `apply-sequence` to `xpath-splitter`

JIRA: https://jira.spring.io/browse/INT-3415

* Add `XPathSplitterParserTests`

XSD Doc Polishing
INT-3448: Add `@Async` test for `@Gateway`

JIRA: https://jira.spring.io/browse/INT-3448
INT-3448: Addressing PR comments
Late Resolution of @Publisher.defaultChannel

Early resolution of the publisher default channel caused
`@ Configuration` factory beans to be instantiated before
Spring Integration bean post processors had been registered.

Use the channel name instead of channel reference and resolve
it when processing the first publish.
INT-3404: Remove `MProducer` from `AIFileSMS`

JIRA: https://jira.spring.io/browse/INT-3404

Previously, `AbstractInboundFileSynchronizingMessageSource` implemented both `MessageProducer` and `MessageSource`.
The `MessageProducer` is really redundant and confusing.
Also, it might produce some side-effects when using JavaConfig.

Provide some other simple but important fix for all `AbstractMessageSource` implementation to invoke `super.afterPropertiesSet()`

Polishing

Add mock bean factory to failing tests.

Revert state assertions in AbstractInboundFileSynchronizingMessageSource;
the fields are state, not arguments, at the time they are being
asserted.
INT-3448: Add `@Async` test for `@Gateway`

JIRA: https://jira.spring.io/browse/INT-3448
INT-3448: Addressing PR comments
Merge branch 'INT-3448' of https://github.com/garyrussell/spring-integration into INT-3448

Conflicts:
	spring-integration-core/src/test/java/org/springframework/integration/configuration/EnableIntegrationTests.java
INT-3448: Polishing