TODO: Optimize routing algorithm

[joemphilips]

When I ported the routing algorithm from eclair, there were two things I left behind.

1. It uses Binary Heap as a data structure to hold the vertex while calculating the route. Original implementation uses Fibonacci Heap, so the time complexity of Dijkstra's algorithm is O(VlogV * E). But our implementation has O(VlogV * ELogV). (where E and V is the number of edges and vertices). this is because Fibonacci heap is more hard to implement than other more simple Heap type, and I couldn't find reference implementation in F#.
2. I didn't care much about converting ResizeArray -> seq -> list when
   finding k-cheapest path by yenKShortestPath . It might improve the performance by not converting intermediate data to other data type.