ratelimited slack client.conversations_list poorman solution

[nzin-alayacare]

Description

When configuring a Slack Bot, Onyx try to list all public/private slack channels, to let you propose on which slack channel(s) you want to configure Onyx.
The problem is that on big Slack workspace (in my case there are > 4000 public/private slack channels), the client.conversations_list returns very quickly the first ~ 1000 slack channels, but face a rate limiting issue after that.

The proposed solution is not a prodution-ready solution, but it is a "poor man" solution:

• if the time to get back the list of all channels is taking more than 10s we failed (we returns a 202)
• BUT we start a background thread, to continue a "cold start" job to collect all channels
• when we have finished to collect all channels, we will be able to serve the saved-for-later list, next time someone ask for all channels

How Has This Been Tested?

It was tested on our company workspace, which contains > 4000 slack channels. It "works" but the cold start takes ~ 30 minutes to get the list of all channels.

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[nzin-alayacare]

cf #4095