Previously if two message requests appered in a short timespan
in the same channel, the bot would reply twice (in two messages)
for the first message.
This commit fixes that, by queuing message requests and responding
to them in the chronological order based on a channelId.
(requests are queued to a queue identified by channelId)
fixes#4
Previously, when OpenAI returned a message with an empty string,
it would try to send that empty message, which throwed an error.
Now it will react with an emoji to the message that triggered the request.
