[DPSLN-2] SimpleLocalnet should remove peers that no longer exist

[qnikst]

[Imported from JIRA. Reported by Edsko de Vries @edsko) as DPSLN-2 on 2012-11-07 15:33:22]
This is not a major issue, because even if we do, there is still no guarantee that a peer still exists after it has been returned by findPeers.