Internal Channel state architecture

[optout21]

How to keep channel state during the splicing process? Several alternatives:

• Add a few fields (new struct member to channel.pending_splice), and keep splicing state there (items such as new capacity, new funding txid, etc.)

• Add a full Channel struct to the Channel, used during the splicing process

• Maintain a separate Channel instance next to the old one (different temp ID), linked by channel ID references

Also has implications for persistence.

Also related to how channel state is used (#8), and #5 and #6.

[optout21]

Note: I've just realized that @ddustin's https://lightningsplice.com/splicing_explained.html site discusses exactly this issue :)