irc: Mark Redis2Irc.privmsg_many as async
Update Redis2Irc.privmsg_many's signature to mark it as async and make it await each call to Redis2Irc.privmsg.
Only async
calls should be enqueued as asyncio.Task objects.
Attempting to enqueue a synchronous call will raise a potentially
confusing "a coroutine was expected, got None" exception.
Bug: T359097