JIRA: https://jira.spring.io/browse/INT-3387,
https://jira.spring.io/browse/INT-3806
* Introduce
```
MessageGroupStore
void addMessagesToGroup(Object groupId, Message<?>... messages);
```
And implement it in all stores.
* Use new `addMessagesToGroup` where it is reasonable, e.g. `DelayHandler`
* Optimize test-case to use a new store method (where it is possible)
* Fix timing delays in the `JdbcMessageStoreTests`
* Introduce `PersistentMessageGroup`
* Add `AbstractMessageGroupStore#proxyMessageGroupForLazyLoad` to wrap the raw `MessageGroup` to the `PersistentMessageGroup` for lazy-load
* Rework `MessageGroupMetadata` do not be `immutable` and allow to store/restore in the `AbstractKeyValueMessageStore` only the `MessageGroupMetadata`
* Refactor `ResequencingMessageHandler` and `SequenceSizeReleaseStrategy` a bit for better performance when interact with the `MessageGroup`
* Add `AbstractMessageGroupStore#setLazyLoadMessageGroups` to switch off the `lazy-load` behavior and restore the previous full `MessageGroup` logic
* Add `What's New` note and `message-store.adoc` paragraph for the lazy-load functionality
`GroupType.PERSISTENT` and not lazy by default
PR Comments
Fix `JdbcMessageStoreTests` timing issues
Address PR comments
* Add performance test to the `ConfigurableMongoDbMessageGroupStoreTests`
* Add JavaDocs for the `MessageGroupFactory` methods
* Add `log4j.properties` into the `test` MongoDB module for better traceability
* Fix `JdbcMessageStore#getOneMessageFromGroup()` over the `doPollForMessage()` delegation.
The `jdbcTemplate.queryForObject()` requires exactly one and only one raw in `resultSet`
* Add performance test results into the `message-store.adoc`
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