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>