Ajeet/graph2

    Graph2, ConstantDB and SimpleMappedGraph
    
    Graph2 is a new base interface for a graph. It lets the neighbors be
    fetched over a network, by wrapping them in a future. In addition, the
    underlying node ids are now longs.
    
    ConstantDB is a port of D. J. Bernstein's excellent cdb package
    (http://cr.yp.to/cdb.html), with a few changes. Unlike cdb, there is
    no limit on the total database size.  A single entry must be smaller
    than 2GB. Additionally, when creating the database the input keys must
    be provided in sorted order.
    
    SimpleMappedGraph is a graph implementation that sits on top of
    ConstantDB and implements Graph2.