Add ChannelContext::get_commitment_stats

It can be useful to get the stats on a potential commitment transaction
without actually building it. Therefore, this commit splits the stats
calculations from the actual build of a commitment transaction.

This introduces an extra loop over the pending htlcs, but current
network behavior produces very few concurrent htlcs on channels.
Furthermore, each iteration of the loop in the stats calculation is very
cheap.

Author: tankyleo