Reduce Heroku worker thread count

[mariusandra]

Changes

• Extracted from Plugin v8 #1946
• Reduce the number of Heroku workers to follow the WEB_CONCURRENCY setting set by resource type

This reduces the number of concurrent threads used in a worker on a "hobby" dyno from 8 to 2, greatly reducing the memory/swap usage:

It also reduces the number of redis connections needed from ~19 when idle to ~15.

Checklist

• All querysets/queries filter by Organization, Team, and User (if this PR affects ANY querysets/queries).
• Django backend tests (if this PR affects the backend).
• Cypress end-to-end tests (if this PR affects the frontend).

[timgl]

I'm not sure why, but when this was deployed the worker wasn't turned on and I had to manually turn it on so no events where coming in. I don't know if that was a direct result of this PR or not,but haven't seen this on other PRs.

[mariusandra]

Indeed the worker didn't come up in my test. #2097

Testing if that's also the case with a fresh branch from master: #2099

[mariusandra]

The same happened with a fresh app deployed from master!

• Test Heroku Worker Deployment #2099
• https://dashboard.heroku.com/apps/posthog-heroku-worker-t-kxgoto

Not sure why though :/

[mariusandra]

Every branch now deploys without workers started. I don't know why, but will debug. This PR should be safe to merge though.

[mariusandra]

@timgl the worker is deployed when just deploying a new heroku app (followed the docs to get here)

I'll investigate and try to fix this in a new PR and/or make a new issue. Any reason this one should not be merged?

Good news is that redis is quite stable now on an empty instance: