SimpleLocalnet should remove peers that no longer exist

[edsko]

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.

[hyperthunk]

Setting the target release as "some time in the future" as this "is not a major issue". Feel free to push it back into 0.2.0 if you want though.

[mboes]

This issue was moved to #393