Fix blinded path serialization in `Route`

Fix blinded path serialization in `Route`

`Route`'s blinded_path serialization logic writes a blinded path
`Option` per path hop, however on read we (correctly) only read one
blinded path `Option` per path. This causes serialization of
`Route`s with blinded paths to fail to round-trip.

Here we fix this by writing blinded paths per path.