Close DB map in DB server thread

[soininen]

This PR fixes a Traceback that could happen when calling SpineDBServerBase.close_db_map().

Checklist before merging

• Documentation (also in Toolbox repo) is up-to-date
• Release notes have been updated
• Unit tests have been added/updated accordingly
• Code has been formatted by black & isort
• Unit tests pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.15%. Comparing base (5b487d9) to head (a7094be).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #626   +/-   ##
=======================================
  Coverage   86.15%   86.15%           
=======================================
  Files          83       83           
  Lines       11477    11477           
  Branches     1636     1636           
=======================================
  Hits         9888     9888           
  Misses       1238     1238           
  Partials      351      351           

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