[avalanche] Extract out a method to add or update a node

Summary:
The current PeerManager::addNode() API fetches the peer, checks the
delegation, adds or updates the node, then updates the node count on the
peer. The last 2 operations are extracted in another private method so
it can be called independently. This will be used for delaying node
addition when the peer proof has not been discovered yet.

Depends on D9699.

There is no change in behavior.

Ref T1634.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1634

Differential Revision: https://reviews.bitcoinabc.org/D9698