bLIP-52/LSPS2 service: Retry failed channel openings

[tnull]

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

While it's working, we do not yet track pending requests and retry if we were to fail the first channel opening attempt. We need to add such logic to make the flow more resilient.

It's a bit unclear as of now though what parts of such retry logic should live here, and what parts could live in lightning-liquidity directly.