[8.x] Limit badger gc concurrency to 1 to avoid panic (backport #14340)

[mergify]

Motivation/summary

Badger GC will panic when run concurrently. 2 TBS processors may run concurrently during a hot reload. Make TBS processor concurrent-safe by protecting badger gc using a mutex.

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

See unit test

Alternatively, somehow (by modifying code locally?) trigger a slow EA reload, and observe that despite 2 TBS processors are running concurrently, gc never runs concurrently.

Related issues

Fixes #14305

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This is an automatic backport of pull request #14340 done by [Mergify](https://mergify.com).