Provide access to stored messages from the ChannelMessageStore API

[mrnst]

It should be possible to use the public API of ChannelMessageStore to get access to stored messages without consuming those messages.

The current implementation always yields an empty message group:

/**
 * Not fully used. Only wraps the provided group id.
 */
@Override
public MessageGroup getMessageGroup(Object groupId) {
    return getMessageGroupFactory().create(groupId);
}

[artembilan]

While implementing #3446 I've noticed that MongoDbChannelMessageStore extends AbstractConfigurableMongoDbMessageStore with its lazyLoadMessageGroups = true by default and give us a full access to the stored messages, either via getMessagesForGroup(Object groupId) or new streamMessagesForGroup(Object groupId).

So, we probably need just pull a couple MessageGroupStore method up into BasicMessageGroupStore or have their respective implementation in the ChannelMessageStore. And also revise a messageGroupFactory to be lazy for the JdbcChannelMessageStore and RedisChannelMessageStore, respectively.

Although it's probably still won't be so straightforward without any breaking changes. So, still 6.0.