Change variable nomenclature for to_self_delay

Change variable nomenclature for to_self_delay

To avoid reviewers confusion, rename counterparty_to_self_delay
to counteparty_selected_contest_delay, i.e the justice delay announced
by a channel counterparty restraining our transactions, and to_self_delay
to locally_selected_contest_delay, i.e the justice delay announced by us
restraining counterparty's transactions

We deviate from wider nomenclature by prefixing local data with a
locally_ extension due to the leak of this value in transactions/scripts
builder, where the confusion may happen.

Rename further AcceptChannelData to the new nomenclature.