userTSDB: Avoid race condition when setting db

[aknuds1]

What this PR does

Fix race condition in pkg/ingester.userTSDB when setting its db field.

WARNING: DATA RACE
Read at 0x00c0153be140 by goroutine 76199:
  github.com/grafana/mimir/pkg/ingester.(*userTSDB).blocksToDelete()
      /Users/arve/Projects/grafana/backend-enterprise/vendor/github.com/grafana/mimir/pkg/ingester/user_tsdb.go:364 +0x58
  github.com/grafana/mimir/pkg/ingester.(*userTSDB).blocksToDelete-fm()
      <autogenerated>:1 +0x51
  github.com/prometheus/prometheus/tsdb.(*DB).reloadBlocks()
      /Users/arve/Projects/grafana/backend-enterprise/vendor/github.com/prometheus/prometheus/tsdb/db.go:1654 +0x49e
  github.com/prometheus/prometheus/tsdb.(*DB).run()
      /Users/arve/Projects/grafana/backend-enterprise/vendor/github.com/prometheus/prometheus/tsdb/db.go:1149 +0x771
  github.com/prometheus/prometheus/tsdb.open.gowrap1()
      /Users/arve/Projects/grafana/backend-enterprise/vendor/github.com/prometheus/prometheus/tsdb/db.go:1093 +0x4f

Previous write at 0x00c0153be140 by goroutine 5042:
  github.com/grafana/mimir/pkg/ingester.(*Ingester).createTSDB()
      /Users/arve/Projects/grafana/backend-enterprise/vendor/github.com/grafana/mimir/pkg/ingester/ingester.go:2717 +0x2015
  github.com/grafana/mimir/pkg/ingester.(*Ingester).openExistingTSDB.func1()
      /Users/arve/Projects/grafana/backend-enterprise/vendor/github.com/grafana/mimir/pkg/ingester/ingester.go:2818 +0x116
  golang.org/x/sync/errgroup.(*Group).Go.func1()
      /Users/arve/Projects/grafana/backend-enterprise/vendor/golang.org/x/sync/errgroup/errgroup.go:78 +0x91

Which issue(s) this PR fixes or relates to

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• about-versioning.md updated with experimental features.