Internal Channel state handling

[optout21]

How to deal with internal channel State during the Splicing process? Options:

• Keep it in ChannelReady, as the 'old' channel does not change and in fact it is usable

• Go through the FundingCreated, FundingSend, ChannelReady, etc. sequence, similar how it happens during the regular open, to reflect the process of the 'new' channel

• A combination: keep ChannelReady, but for the splicing process have a separate splicing state field, used during the splicing process where appropriate

[optout21]

Note: the prototype (up to current hapa4) cycles through the states, to satisty checks in some codepaths that are executed in both scenarios (regular open and splicing), but that can be changed