Consider using AnyCable for websockets

Description

I couldn't find any discussion about websockets in GitLab.

Evil Martians published some time ago AnyCable (Action Cable on steroids)

1. Blog post: https://evilmartians.com/chronicles/anycable-actioncable-on-steroids
2. Anycable: https://github.com/anycable/anycable
3. Anycable Go WebSocket Server: https://github.com/anycable/anycable-go
4. Anycable Rails: https://github.com/anycable/anycable-rails

Broadcasting performance (the image from the blog post):

Requirements

Rails >= 5.0
Ruby >= 2.3, <= 2.5

Proposal

Consider adding AnyCable to a heavy part of GitLab (CI build log output?) hidden by a feature flag.
Check it on production how it works and whether it's worth to move forward with AnyCable.

/cc @stanhu @DouweM @rymai @ayufan @smcgivern

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