[avalanche] Remove getPubkey API

Summary: Instead, provide a more generic forNode function that provides a temporary access to the node object. This ensure locking remains correct while not requiring accessor for anythign and everything.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6828