backend: delete dead clients from epoll #1229

Job	Run time
cargo-deny	37s
clippy-check	1m 15s
format	38s
coverage-main	1m 6s
Documentation on Github Pages	1m 3s
coverage-tests	1m 33s
check-minimal	1m 7s
coverage-tests (server_system)	1m 19s
msrv	2m 42s
coverage-tests (client_system)	1m 12s
coverage-tests (client_system, server_system)	1m 4s
publish	39s
	14m 15s

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