INT-2460: Remove Message Modification Logic in MS

JIRA: https://jira.spring.io/browse/INT-2460,
https://jira.spring.io/browse/INT-4122

Since the main purpose of the `MessageStore` to persist message for durability and only,
it doesn't make sense to modify `Message` for additional headers like `SAVED` and `CREATED_DATE`.
Such a logic should be a part of metadata stored together with the message.
And it is provided by the out-of-the-box `MessageStore` implementation.
In addition we free ourselves from the reflection operations to retain `ID` and `TIMESTAMP` headers when we add `SAVED` and `CREATED_DATE`

* Control "already saved" logic in the `JdbcMessageStore`s via `DuplicateKeyException` on the `INSERT`.
This is much effective then additional `SELECT` in case of `SAVED` before
* Control "already saved" logic in the  `AbstractConfigurableMongoDbMessageStore` via `DuplicateKeyException` on the `INSERT`.
Since `MongoDbMessageStore` doesn't provide extra `messageId` field, perform extra `SELECT` before store document.
Anyway the `MongoDbMessageStore` isn't recommended for use.
We may consider to deprecate it
* Control "already saved" logic  in the `AbstractKeyValueMessageStore` via `putIfAbsent` operation

With this fix we persist message in the store as is without any modifications when we perform standard serialization procedure.
Any custom serializers should consider to use `MutableMessageBuilder` if there is a requirement to retain `ID` and `TIMESTAMP`

Rework `MongoDbMetadataStore.putIfAbsent()` to normal `findAndModify()` with particular `$setOnInsert`.
Technically the MongoDB query looks like:
```
db.collection.findAndModify({
  query: { _id: $key },
  update: {
    $setOnInsert: { value: $value } // perform modification only on upsert
  },
  new: false,   // don't return new doc if one is upserted
  upsert: true // insert the document if it does not exist
})
```

Move single import to the appropriate JavaDoc

Polishing after rebase
DEBUG messages in `doStoreIfAbsent()` implementations

* To keep track of the extra message information in the `MessageStore`, without `Message` modification, introduce `MessageMetadata` and `MessageHolder`
* Add `MessageStore#getMessageMetadata()`
* Modify MongoDb `MessageStore` to add extra `timestamp` for individual message
* Fix `ConcurrentAggregatorTests` race condition.
Since currently the default release strategy is `SimpleSequenceSizeReleaseStrategy` which is just based on the `MessageGroup` size, there is no guaranty which messages will complete the group in concurrent environment.
The test is really based on the `SequenceAwareMessageGroup` logic to discard the message with the same `correlationId`

Fix `@Copyright` format

Move cast to `MessageHolder` after `Assert.isInstanceOf(MessageHolder.class, messageHolder)`

Retain backward compatibility in the `AbstractKeyValueMessageStore`

Polishing

spring-integration-gemfire/src/main/java/org/springframework/integration/gemfire/store/GemfireMessageStore.java

@@ -106,6 +106,17 @@ public class GemfireMessageStore extends AbstractKeyValueMessageStore implements
 		this.messageStoreRegion.put(id, objectToStore);
 	}
 
+	@Override
+	protected void doStoreIfAbsent(Object id, Object objectToStore) {
+		Assert.notNull(id, "'id' must not be null");
+		Assert.notNull(objectToStore, "'objectToStore' must not be null");
+		Object present = this.messageStoreRegion.putIfAbsent(id, objectToStore);
+		if (present != null && logger.isDebugEnabled()) {
+			logger.debug("The message: [" + present + "] is already present in the store. " +
+					"The [" + objectToStore + "] is ignored.");
+		}
+	}
+
 	@Override
 	protected Object doRemove(Object id) {
 		Assert.notNull(id, "'id' must not be null");