improve query cancellation logic for server

[dhirving]

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes
• (if changing dimensions.yaml) make a copy of dimensions.yaml in configs/old_dimensions

[codecov]

Codecov Report

Attention: Patch coverage is 70.58824% with 10 lines in your changes missing coverage. Please review.

Project coverage is 89.53%. Comparing base (7efb63b) to head (087c097).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...n/lsst/daf/butler/registry/interfaces/_database.py	37.50%	5 Missing ⚠️
...r/remote_butler/server/handlers/_external_query.py	86.36%	3 Missing ⚠️
...n/lsst/daf/butler/registry/databases/postgresql.py	50.00%	1 Missing ⚠️
...ython/lsst/daf/butler/registry/databases/sqlite.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1067      +/-   ##
==========================================
- Coverage   89.54%   89.53%   -0.02%     
==========================================
  Files         359      359              
  Lines       46509    46532      +23     
  Branches     9566     9570       +4     
==========================================
+ Hits        41648    41661      +13     
- Misses       3511     3521      +10     
  Partials     1350     1350              

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