[avalanche] Change the addavalanchenode RPC so that it return if the operation succeeded

Summary: Also rename avalanche::Processor::addPeer into addNode for consistency.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6911