Expose a claimable `Balance` for no-preimage-but-may-learn-one

[TheBlueMatt]

If we have a pending inbound HTLC which hit the chain, but don't have a preimage for it, we don't generate a Balance for it. This makes sense, generally, but we actually could learn the preimage later if we forwarded the HTLC and the upstream node claimed it from us. Thus, we have to generate some kind of Balance until after the HTLC is irrevocably resolved, but probably a new form.

Tagging 111 cause its important work, but I really don't want to delay the other fixes in 110 for it.