INT-3041 Add Namespace Support For Retry Advice

JIRA: https://jira.spring.io/browse/INT-3041

Add namespace support to simplify configuration of
a `RequestHandlerRetryAdvice.

INT-3041 Polishing; PR Comments

* Allow a retry-advice element within the request-handler-advice-chain
* Clean up schema (should not have allowed `synchronization-factory` on
    the request-handler-advice-chain.

INT-3041 Polishing; PR Comment