Further simplify LSPS2 client-side flow

We further simplify the LSPS2 client-side flow by:

- Only track state starting in `BuyRequested`
- Have `select_opening_params` take the corresponding arguments
- Drop `jit_channel_id` for tracking everything by `user_channel_id`
- Cleanup peer state by removing accessors and always using
  `HashMap`/`HashSet` methods directly

Author: tnull