[Peers] Tech Debt: Rename node_id/their_node_id

[julianknutsen]

As a developer, I want variable names to be consistent across the codebase so that it is easier to jump around and understand the code.

Definition of Done:

• decide on name (counterparty_id/counterparty_node_id?)
• do the rename

Tech Discussion:
@ariard suggested counterparty_node_id

[TheBlueMatt]

As of #633 the "correct" answer is counterparty/holyder for them/us and countersignatory/broadcaster for transactions (which have an "owner" which may or may not be us).