If we don't query a server (because we already know about it)

If we don't query a server (because we already know about it)
the "last seen time" will never be updated, so the server will
be discarded.

It seems that we should always ping servers (so that set_seen gets
called on receipt of the response), no matter whether
"auto-discovered" or configured, so that the "discard" code doesn't
kick in.

Otherwise we remove and re-add our configured servers every
10 seconds, which is inefficient and which possibly triggers
other bugs.