SIANXSVC-1060: stability improvments v2

[PatrickTaibel]

Second batch of stability improvements. Those were necessary due to additional problems that occurred during testing.
This should be the final one. The main change is that I rewrote our transaction logic to hopefully not contain any race conditions anymore.

[codecov-commenter]

Codecov Report

Attention: 56 lines in your changes are missing coverage. Please review.

Comparison is base (0828cbb) 42.78% compared to head (795f524) 42.25%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           anexia       #8      +/-   ##
==========================================
- Coverage   42.78%   42.25%   -0.53%     
==========================================
  Files          17       17              
  Lines        1587     1602      +15     
==========================================
- Hits          679      677       -2     
- Misses        857      874      +17     
  Partials       51       51              

Files	Coverage Δ
connection/connection_pool.go	78.02% <0.00%> (-1.76%)	⬇️
server.go	16.66% <0.00%> (-0.31%)	⬇️
connection/connection.go	66.91% <46.15%> (+2.37%)	⬆️
client.go	16.92% <0.00%> (-1.62%)	⬇️

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