MINOR: tcpcheck: Don't handle anymore in-progress connect rules in tcpcheck_main

MINOR: tcpcheck: Don't handle anymore in-progress connect rules in tcpcheck_main

The special handling of in-progress connect rules at the begining of
tcpcheck_main() function can be removed. Instead, at the begining of the
tcpcheck_eval_connect() function, we test is there is already an existing
connection. In this case, it means we are waiting for a connection
establishment. In addition, before evaluating a new connect rule, we take
care to release any previous connection.