fix: disable dashboard insights refresh chaining

[anirudhpillai]

Problem

Temporary fix for https://posthoghelp.zendesk.com/agent/tickets/15410
Where one broken insight on the dashboard prevents entire dashboard from refreshing as insights are refreshed one at a time

Changes

Disable 'chaining' for dashboard insight refresh.
When /dashboard refresh is triggered, all the insights on it are refreshed in parallel (instead of sequentially before), so one slow insight won't hold back the other insight.

Impact is more async queries per dashboard. But we still limit concurrency per team here

Caveats

1. The UI still polls 2 insights at a time so if 2 broken insights on a dashboard are polled first then they'll both prevent the UI from reading updated results for other insights. Can increase this limit?
2. Haven't disabled chaining for shared dashboards yet but would make sense to disable that as well.

Does this work well for both Cloud and self-hosted?

N/A

How did you test this code?

Tested locally by replicating user scenario.

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ CHQueryErrorTimeoutExceeded: DB::Exception: Timeout exceeded: elapsed 91.684698318 seconds, maximum: 60. Stack trace: /api/projects/{parent_lookup_team_id}/dashboard... View Issue

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