Fixing https://github.com/tomp2p/TomP2P/issues/25. The problem was that
a peer that went offline and came back, was marked offline in the other
peers. The offline marked peer then never has a chance to be in the
maintenance queue. Now if a peer comes back, it will land in the
maintenance queue and may come back online. Also added the notion of
second-hand peer and third-hand peer.
Added example related to https://github.com/tomp2p/TomP2P/issues/25.