Don't bump the `next_node_counter` when using a removed counter

Don't bump the `next_node_counter` when using a removed counter

If we manage to pull a `node_counter` from `removed_node_counters`
for reuse, `add_channel_between_nodes` would `unwrap_or` with the
`next_node_counter`-incremented value. This visually looks right,
except `unwrap_or` is always called, causing us to always increment
`next_node_counter` even if we don't use it.

This will result in the `node_counter`s always growing any time we
add a new node to our graph, leading to somewhat larger memory
usage when routing and a debug assertion failure in
`test_node_counter_consistency`.

The fix is trivial, this is what `unwrap_or_else` is for.