Figure out path-length-limiting when routing with metadata/blinded paths

[TheBlueMatt]

Now that we support payment metadata, which reduces the max path length, and given we're about to support blinded paths, we need to figure out the max path length more carefully when routing.

[TheBlueMatt]

This is important to fix, but it doesn't need to be fixed for MVP of BOLT12, slipping to 117.