Improve ChannelDetails readability significantly.

Improve ChannelDetails readability significantly.

After the merge of #984, Jeff pointed out that `ChannelDetails` has
become a bit of a "bag of variables", and that a few of the variable
names in #984 were more confusing than necessary in context.

This addresses several issues by:
 * Splitting counterparty parameters into a separate
   `ChannelCounterpartyParameters` struct,
 * using the name `unspendable_punishment_reserve` for both outbound
   and inbound channel reserves, differentiating them based on their
   position in the counterparty parameters struct or not,
 * Using the name `force_close_spend_delay` instead of
   `spend_csv_on_our_commitment_funds` to better communicate what
   is occurring.