Add a new `IndexedMap` type and use it in network graph storage

Add a new `IndexedMap` type and use it in network graph storage

Our network graph has to be iterable in a deterministic order and
with the ability to iterate over a specific range. Thus,
historically, we've used a `BTreeMap` to do the iteration. This is
fine, except our map needs to also provide high performance lookups
in order to make route-finding fast. Sadly, `BTreeMap`s are quite
slow due to the branching penalty.

Here we replace the `BTreeMap`s in the scorer with a dummy wrapper.
In the next commit the internals thereof will be replaced with a
`HashMap`-based implementation.