INT-2879: JPA Retrieving: Add `id-expression`

JIRA: https://jira.spring.io/browse/INT-2879

* Add `id-expression` attribute for the `<int-jpa:retrieving-outbound-gateway>`
* Make it mutually exclusive with other `SELECT` attributes
* Remove deprecated `max-number-of-results`

INT-2879 Polishing

- Improve parser error message.
- Minor doc polish.

INT-2879: Support `delete-after-poll` and `flush`
INT-3378 Bump Spring-AMQP to 1.3.2

JIRA: https://jira.spring.io/browse/INT-3378

Adds a default error handler that rejects messages that
can't be converted and would cause an infinite loop.
INT-3382 Support Easier Aggregator Extensions

JIRA: https://jira.spring.io/browse/INT-3382

- Make private methods protected; add protected getters
- Support obtaining just metadata from stores
- Support just fetching the first message from a group
- add metaSize() to message group