[avalanche] More bucket brigading removal in the Processor

Summary: This time, focused on FindProofForGetData. It now seems that there is a bunch of crap that is only used by tests and that the API can be dramatically reduced both in the Processor and PeerManager.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9749