Adapt backend to custom zhinst.comms package

[tobiasah]

This commit replaces the dependency to pycapnp with zhinst.comms. This
fixes the stability issues we had before and also allows us to simplify
the python code.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.64353% with 36 lines in your changes missing coverage. Please review.

Project coverage is 95.81%. Comparing base (7cf3492) to head (afc3798).

Files	Patch %	Lines
src/labone/core/subscription.py	64.10%	12 Missing and 2 partials ⚠️
src/labone/core/session.py	88.88%	7 Missing and 1 partial ⚠️
src/labone/server/server.py	83.78%	4 Missing and 2 partials ⚠️
src/labone/core/value.py	88.88%	3 Missing and 2 partials ⚠️
src/labone/core/kernel_session.py	88.23%	1 Missing and 1 partial ⚠️
src/labone/server/session.py	98.14%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #63      +/-   ##
==========================================
- Coverage   99.26%   95.81%   -3.46%     
==========================================
  Files          34       26       -8     
  Lines        1915     1432     -483     
  Branches      295      230      -65     
==========================================
- Hits         1901     1372     -529     
- Misses         14       48      +34     
- Partials        0       12      +12     

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