add retuning 'int' with redis publish

[nihiyama]

I think there is logic that I want to determine if it is subscribed and process it.
For example, if I don't have any subscribers, I want to discard or retry the publish.

I didn't know if a publish other than redis would return an int, so I made it the Optional[int] type in the Broadcast class and the int type in the RedisBackend class.

If all the subscribers are disconnected, I want the publish return to be 0, so I'd like to merge this pull request as well.