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/linkgraphjob_base.h

@@ -16,6 +16,6 @@
 
 typedef LinkGraphJob::Node Node;
 typedef LinkGraphJob::Edge Edge;
-typedef LinkGraphJob::EdgeIterator EdgeIterator;
+typedef LinkGraphJob::AnnoEdge AnnoEdge;
 
 #endif /* LINKGRAPHJOB_BASE_H */