ipa-replica-conncheck: do not close listening ports until required
    
    Previously, a separate thread would be created for each socket used
    for conncheck. It would also time out after one second, after which it
    would be closed and reopened again. This caused random failures of
    conncheck.
    
    Now all sockets are handled in a single thread and once the server
    starts to listen on a port, it does not close that connection until the
    script finishes.
    
    Only IPv6 socket is used for simplicity, since it can handle both IPv6
    and IPv4 connections. This requires IPv6 kernel support, which is
    required by other parts of IPA anyway.
    
    https://fedorahosted.org/freeipa/ticket/6487
    
    Reviewed-By: Petr Spacek <pspacek@redhat.com>