The purpose of this commit is to begin the process of packing
symmetric fields into the newly introduced Dual structure. The
reason for this is that the Dual structure has a handy indexing
method where we can supply a ChannelParty and get back a value.
This will cut down on the amount of branching code in the main
lines of the codebase logic, making it easier to follow what is
going on.
In this commit, we start to use the new AuxSigner to obtain+verify aux sigs for all second level HTLCs. This is similar to the existing SigPool, but we'll only attempt to do this if the AuxSigner is present (won't be for most channels).
In this commit, we add a new aux signer interface that's meant to mirror the SigPool. If present, this'll be used to (maybe) obtain signatures for second level HTLCs for certain classes of custom channels.
