bLIP-52/LSPS2 service: Fully implement client-trusts-LSP flow

[tnull]

In #420 we added initial support for acting as an bLIP-52/LSPS2 service.

Currently, we simply hope the client will claim the HTLC after we opened this channel. To avoid potentially having the LSP being the target of griefing attacks, we should implement the full 'client-trusts-LSP' flow, i.e., only broadcast the funding transaction once the client claimed the payment.