python: release GIL when running server

[dlech]

🦟 Bug fix

Fixes #

Summary

Since running the server can be a blocking function, we need to release the GIL so that other Python threads can run. This also means we no longer need to poll for the server to finish running by setting the blocking argument to True.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[codecov]

Codecov Report

Merging #1458 (168e797) into ign-gazebo6 (07bbeb2) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           ign-gazebo6    #1458   +/-   ##
============================================
  Coverage        33.58%   33.58%           
============================================
  Files               44       44           
  Lines             2260     2260           
============================================
  Hits               759      759           
  Misses            1501     1501           

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