Allow sending messages in bulk

[olzhasar]

Feature request

Sometimes there is a need to send multiple messages to a channel (or group) at once. While this can be done using a group of asyncio tasks, the underlying logic will be suboptimal, cause there would be multiple redis calls for discarding old messages, checking the capacity, appending to the set and setting the expiration.

It would be cool if there were optimized functions for that using a single zadd call.
Something like this:

    async def send_multiple(self, channel, messages: list):
        ...

    async def group_send_multiple(self, channel, messages: list):
        ...

I am not sure if such an issue should be created in the channels repo first, let me know if that's the case.

[carltongibson]

This isn't a bad idea. We'd probably need to add something to the base channel layer, and docs too.

Do you want to experiment with an implementation?

[olzhasar]

Sure, I will prepare a PR for this once I have some free time.