Change LinkGraph::EdgeMatrix to a sparse storage format

Use a btree_map instead of a SmallMatrix.
This avoids excessive storage requirements for large graphs,
and overly expensive edge matrix resize operations.
Remove next_edge iteration mechanism.

src/linkgraph/mcf.h

@@ -24,7 +24,7 @@ protected:
 	template<class Tannotation, class Tedge_iterator>
 	void Dijkstra(NodeID from, PathVector &paths);
 
-	uint PushFlow(Edge &edge, Path *path, uint accuracy, uint max_saturation);
+	uint PushFlow(AnnoEdge &edge, Path *path, uint accuracy, uint max_saturation);
 
 	void CleanupPaths(NodeID source, PathVector &paths);