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.
Passengers usually prefer fast paths to short paths.
Average travel times of links are updated in real-time for use in Dijkstra's algorithm,
and newer travel times weigh more, just like capacities.
(cherry picked from commit 977604ef08)