Clear execution metadata and use datetime from message header

[kevin-bates]

No description provided.

[codecov-commenter]

Codecov Report

Merging #1 (c38c54a) into master (f688790) will decrease coverage by 0.00%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master       #1      +/-   ##
==========================================
- Coverage   81.16%   81.15%   -0.01%     
==========================================
  Files          13       13              
  Lines        1439     1449      +10     
==========================================
+ Hits         1168     1176       +8     
- Misses        271      273       +2     

Flag	Coverage Δ
unittests	81.15% <87.50%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
nbclient/tests/test_client.py	99.67% <ø> (+0.32%)	⬆️
nbclient/client.py	90.41% <87.50%> (-0.68%)	⬇️

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