INT-3557 Clarify MessageHeaders Constants

JIRA: https://jira.spring.io/browse/INT-3557

Add class names and code tags.

Add missing header constants.

Polishing for `timestamp` header doc.

Fix JavaDocs issues: https://build.spring.io/browse/INT-EIGHTAPI-296
Fix Docs issue

https://build.spring.io/browse/INT-B41-152
INT-3531 Update to Spring Framework 4.1.2

JIRA: https://jira.spring.io/browse/INT-3531
INT-3531 Fix Stub AMQP Channel

Compatibility with 3.4.x amqp-client.