backend: delete dead clients from epoll #1228

Job	Run time
cargo-deny	40s
clippy-check	1m 3s
format	24s
coverage-main	1m 30s
check-minimal	51s
Documentation on Github Pages	57s
coverage-tests	1m 33s
coverage-tests (server_system)	1m 30s
msrv	2m 22s
coverage-tests (client_system)	1m 46s
coverage-tests (client_system, server_system)	1m 49s
publish	42s
	15m 7s

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