Correctly handle asynchronously re-established server links

Correctly handle asynchronously re-established server links

Don't try to establish an outgoing server link after DNS lookup when this
server re-connected on its own in the meantime.
In addition, log a warning message if we try to update the connection
index of an already connected server structure -- and ignore it.

Up to now, both behaviour could lead to a race when the remote server
connects to this daemon while it still prepares the outgoing connection:

 - The local server prepares the new outgoing connection ...
 - in the meantime the remote server becomes connected and registered.
 - Now the new outgoing connection overwrites the (correct) socket handle,
 - then the 2nd connection becomes disconnected: "already registered",
 - and the 1st connection becomes unhandled ("gets lost") because the
   configuration structure is reset because of the wrong socket handle.

This patch hopefully fixes all these problems.