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).
This commit breaks the ChannelConstraints structure into two
sub-structures that reflect the fundamental differences in how
these parameters are used. On its face it may not seem necessary,
however the distinction introduced here is relevant for how we
will be implementing the Dynamic Commitments proposal.
