INT-3148: Add MetadataStore Chapter

JIRA: https://jira.springsource.org/browse/INT-3148

* Add a general chapter about `MetadataStore`
* Add cross-links between chapters on the matter of `MetadataStore`

Doc Polishing
INT-3149: Cleanup of the What's New

JIRA: https://jira.springsource.org/browse/INT-3149

* Add a note about `GatewayMethodMetadata`
* Squash several sections to one by type
* Reorder by importance/interestingness
* Remove a note about `MessagingException` for `RetryAdvice`,
as the last one doesn't wrap exceptions to `MessagingException` on each retry