INT-3075: Note regarding HTTP Status Code

JIRA: https://jira.springsource.org/browse/INT-3075
INT-3138: add `awaitTermination` to delayer tests

`DelayerHandlerRescheduleIntegrationTests` fail sometimes
on asserts. Looks like `context.destroy()` works very slow
and `<delayer>` manages to send delayed Messages

JIRA: https://jira.springsource.org/browse/INT-3138
INT-3092, INT-3042: Doc: Jacksons, Resequencer

JIRA: https://jira.springsource.org/browse/INT-3042
https://jira.springsource.org/browse/INT-3092
INT-3027: Headers Enrichment for <enricher>

* Add `<header>` sub-element to `<enricher>`
* Refactoring to `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