backend: delete dead clients from epoll #1227

Job	Run time
format	27s
cargo-deny	31s
clippy-check	1m 13s
coverage-main	1m 46s
Documentation on Github Pages	49s
coverage-tests	1m 42s
check-minimal	43s
coverage-tests (server_system)	1m 35s
msrv	3m 14s
coverage-tests (client_system)	1m 59s
coverage-tests (client_system, server_system)	1m 30s
publish	29s
	15m 58s

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