fix(grpc): more problems with UpdateObject()/UpdateBucket()

[coryan]

I thought my previous tests had reproduced and fixed the problem, but
evidently this is not the case. It all seems to be working correctly
now, tested using the C++ client library. I also filed a bug against
the protobuf project, and another bug to cleanup this code when the
protobuf bug is fixed.

See #270 for more details.

[codecov]

Codecov Report

Merging #271 (e16dac3) into main (9a145b0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #271   +/-   ##
=======================================
  Coverage   97.61%   97.61%           
=======================================
  Files          48       48           
  Lines        7582     7598   +16     
=======================================
+ Hits         7401     7417   +16     
  Misses        181      181           

Flag	Coverage Δ
unittests	97.61% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
testbench/grpc_server.py	100.00% <100.00%> (ø)
tests/test_grpc_server.py	99.84% <100.00%> (+<0.01%)	⬆️

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