* Introduce a `groupConditionSupplier` for MGS
* Add a `MessageGroup.condition` option
* Add a `MessageGroupStore.conditionSupplier` option
* Use it from the `SimpleMessageStore.addMessagesToGroup()` API
to populate `condition` (if any) into a `MessageGroup`
* Introduce a `GroupConditionProvider` contract to be implemented
on those `ReleaseStrategy` contracts which could be aware of group condition
* Populated a `GroupConditionProvider.getGroupConditionSupplier()`
into a `MessageGroupStore` from the `AbstractCorrelatingMessageHandler`
for end-user convenience
* Rework a `FileMarkerReleaseStrategy` to implement a `GroupConditionProvider`
to provide a function which produces a condition from `file_lineCount` header
of the `END` marker message
* Make the `FileMarkerReleaseStrategy` logic already based on the condition from a group
* Delegate `GroupConditionProvider` from the `FileAggregator`
* Add test for empty file aggregation
* * Implement `condition` in the `AbstractKeyValueMessageStore` and `MongoDbMessageStore`
* Test `condition` for `mongo-aggregator-config.xml` and `FileAggregatorTests` against GemFire
* * Implement `condition` in the `ConfigurableMongoDbMessageStore`
* * Implement `condition` in the `JdbcMessageStore`
* `FileAggregatorTests` against `JdbcMessageStore`
* Refactor `JdbcMessageStore` for better handling of message group metadata
* Remove unused `MARKED` column in the DDL in favor of newly introduced `CONDITION`
* * Add docs for message group condition
* * Move `conditionSupplier` option from MGS to AbstractCorrelatingMH
* Make it as a `BiFunction` to propagate existing condition alongside with the
message to consult
* Expose `groupConditionSupplier` in Java & XML DSLs
* * Fix language in docs
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