We have the issue of aliases: xpay uses scids like 0x0x0 for
routehints and blinded paths, and then can apply reservations to them. But
generally, reservations are *global*, so we need to differentiate.
Changelog-Added: Plugins: `askrene-reserve` and `askrene-unreserve` can take an optional `layer` inside `path` elements.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>