INT-3146 Fix Test Timing Issue

Wait for server connection factory to start listening for new tests.
INT-3027: Headers Enrichment for <enricher>

* Add `<header>` sub-element to `<enricher>`
* Refactoring of `EnricherParser`
* Add Headers Enrichment logic to `ContentEnricher`
* Add tests
* What's new and Content Enricher's `<header>` note

JIRA: https://jira.springsource.org/browse/INT-3027

INT-3027: Addressing PR comments

INT-3027: add NPE asserts to map properties

INT-3027: add overwrite & type to enricher header

Move `HeaderValueMessageProcessor` hierarchy to 'transformer.support' package

INT-3027: Make header's overwrite=true by default

Doc Polishing
INT-3146 Fix Another Test Timing Issue

Increase test timer for soTimeout detection.
INT-3133: Add <spel-property-accessors> Support

* Add <spel-property-accessors> to configure a list of beans
that implement `org.springframework.expression.PropertyAccessor`
* Add `SpelPropertyAccessorRegistrar` to manage a list of PropertyAccessors
for Integration infrastructure
* Refactoring for `SpelFunctionRegistrar` and `IntegrationEvaluationContextFactoryBean`
to fix the issue when there is no `SpelFunctionRegistrar`(`SpelPropertyAccessorRegistrar`)
in the child AC, but there is one in the parent.
Previously the inheritance did't work for that reason.
* Now parent/child logic moved to `IntegrationEvaluationContextFactoryBean`
* Add documentation

JIRA: https://jira.springsource.org/browse/INT-3133

INT-3133: Rebasing, polishing and documentation

INT-3133: PropertyAccessors override support

* `@Ignore` `SOLingerTests#finReceivedNioLinger()` test

INT-3133: Change JavaDoc link to 3.0.0.RC1

Polishing

- Remove duplicate check - last bean def wins
- Doc Polishing