Drop the `dist` `HashMap` in routing, replacing it with a `Vec`.

Drop the `dist` `HashMap` in routing, replacing it with a `Vec`.

Now that we have unique, dense, 32-bit identifiers for all the
nodes in our network graph, we can store the per-node information
when routing in a simple `Vec` rather than a `HashMap`. This avoids
the overhead of hashing and table scanning entirely, for a nice
"simple" optimization win.

Because we must ensure we have a 1-to-1 mapping from node ids to
`node_counter`s, even across first-hop and last-hop hints, we have
to be careful to check the network graph first, then a new
`private_node_id_to_node_counter` map to ensure we only ever end up
with one counter per node id.