Support switching block sync backends

[TheBlueMatt]

We don't really currently support this unless we're fully synced when we last shut down, which isn't really something we track. There's been a lot of discussion on how to do this, but more work to be done. #3600 took a pass at it, though ultimately for switching between bitcoinds if we're stale #3876 made progress and https://github.com/lightningdevkit/rust-lightning/pull/3876#discussion_r2181044391 described the next steps towards finishing at least that.