Derive FundingContribution::net_value implicitly

This commit removes `FundingContribution::value_added` as tracking it is
unnecessary -- it can just be derived from the total amount in minus
total amount out minus fees. Doing so also highlighted that there was an
incorrect assumption in how feerates are computed/adjusted. Ultimately,
we don't care whether a contribution has inputs or not. Instead, we look
at whether it's contributing a positive or negative amount to the
channel, and enforce different constraints for each.

Author: wpaulino