In main serverlistener loop too much time wasted on TLS handshake + request reading
between Accept() connection and start of goroutine handleCheck() in processConnection().

On high RTT links new connections accumulates, in "ss -np | grep 10050" on agent host
you can see a lot of ESTAB connections with 328 bytes in Recv-Q (client hello packet)
and without attached file descriptor in zabbix_agent2 on it.

It leads to timeout on server side, errors in logs and items queue growth.

Moving of TLS handshake and request reading into a separate goroutine fixed this behavior.