backend: delete dead clients from epoll

[Drakulix]

I am not sure, if we need to do the same thing here for kqueue, I only observed this problem on an epoll system, but I figured it can't hurt.

This fixes a deadlock from killed clients blocking the loop in dispatch_all_clients for the rust-backend.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.84%. Comparing base (1f6f4f5) to head (4108eea).
Report is 41 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #741      +/-   ##
==========================================
+ Coverage   73.11%   75.84%   +2.72%     
==========================================
  Files          45       46       +1     
  Lines        8183     8226      +43     
==========================================
+ Hits         5983     6239     +256     
+ Misses       2200     1987     -213     

Flag	Coverage Δ
main	58.90% <100.00%> (-0.02%)	⬇️
test--	81.09% <100.00%> (?)
test--server_system	64.69% <0.00%> (?)
test-client_system-	72.10% <100.00%> (?)
test-client_system-server_system	54.97% <0.00%> (-0.01%)	⬇️

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