Conversations

We need some extra routines to combine connected components and betweenness centrality in directed graphs.  We want to do more with edge types, so I'm adding EDGE
_TYPE and VERTEX_TYPE to the RPC server.  New RPC methods include connected components, breadth-first search, and extracting users adjacent to conversations and conversations adjacent to users.  We still need additional support for edge types (like in betweenness centrality).