[avalanche] Make the proof pool internal container private

Summary:
This adds the missing accessors for keeping the multi index container private.

Ref T1854.

Depends on D10603.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10597