When creating the keyring, the tweak is already calculated in the remote
commitment case. We add the calculation also for our own commitment, so
we can use it in all cases without deriving the tweak.
Based on the current channel type, we derive the script used for the
to_remote output. Currently only the unencumbered p2wkh type is used,
but that will change with upcoming channel types.
To make the channel state machine less concerned about the type of
commitment, we nil the local tweak when creating the keyring, depending
on the commitment type.
We abstract away how keys are generated for the different channel types
types (currently tweak(less)).
Intention is that more of the logic that is unique for each commitment
type lives in commitment.go, making the channel state machine oblivious
to the keys and outputs being created on the commitment tx for a given
channel state.
createCommitmentTx would earlier mutate the passed commitment struct
after evaluating the htlc view and calculating the final balances, which
was confusing since the balances are supposed to only be *after*
subtracting fees.
Instead we take the needed parameters as arguments, and return the final
balances, tx and fee to populate the commitment struct in a proper way.
PURE CODE MOVE:
Moving createCommitmentTx, CreateCommitTx, createStateHintObfuscator,
CommitmentKeyRing, DeriveCommitmentKeys, addHTLC, genHtlcScripts
We move the methods and structs to a new file commitment.go in
preparation for defining all the logic that is dependent on the channel
type in this new file.
