slack-19.0: various backports for vtorc, part 2

[timvaillancourt]

Description

This PR contains backports of a collection of v20/v22 contributions to improve the visibility of the backend database, the topo-efficiency of discovery and the overall safety of VTOrc. Also, the upstream version of key-range support was backported, replacing the un-upstreamed support we added

Related Issue(s)

1. Add api end point to print the current database state in VTOrc vitessio/vitess#15485 (v20 PR)
2. Ensure all topo read calls consider --topo_read_concurrency vitessio/vitess#17276 (v22 PR)
   • Avoid flaky topo concurrency test vitessio/vitess#17407 (test-only v22 PR)
3. [release-19.0] vtorc: require topo for Healthy: true in /debug/health (#17129) vitessio/vitess#17351 (v19 backport of v22 PR)
4. vtorc: fetch all tablets from cells once + filter during refresh vitessio/vitess#17388 (v22 PR)
5. Support KeyRange in --clusters_to_watch flag vitessio/vitess#17604 (v22 PR)

This also included a revert of #457, to replace this code with the upstream variant: vitessio#17604

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @GuptaManan100 to sign the Salesforce Inc. Contributor License Agreement.