Use channel to delay chat.postMessage

[NilsIrl]

Currently, we're sending the message during the response. This is problematic because it is technically a long running tasks and we should respond within 3 seconds: https://api.slack.com/events-api#responding_to_events

[NilsIrl]

Implemented in 369ee23

[NilsIrl]

An issue though is that the loop sending messages is synchronous so only 1 message can be sent at a time.