INT-3551: Idempotent Receiver: add value-strategy

JIRA: https://jira.spring.io/browse/INT-3551

* Rename `MetadataKeyStrategy` -> `MetadataEntryStrategy`
* Add `valueStrategy` to the `MetadataStoreSelector`
* Add `value-strategy` and `value-expression` to the `<idempotent-receiver>`
Oh, never mind - matches namespace.