SimpleLocalnet should remove peers that no longer exist

[mboes]

From @edsko on November 7, 2012 15:33

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.

Copied from original issue: #66

[mboes]

From @hyperthunk on January 4, 2013 16:0

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.

[sopvop]

Other problem is that the list of LocalNodes can grow, and number of Monitor, as reported by getLocalNodeStats field nodeMonitors, grows with each node which was lost and reconnected.

If simplelocalnet is intended to be used as example for tutorials and quick prototypes then this should be mentioned in the docs.

I think that maybe adding releaseNode :: Backend -> NodeId -> IO () would be enough for most users, so they can clean it up if they get errors from someplace else.