p2p: Add ReadAnchors()
Summary:
This is a backport of core#17428 [2/7]
https://github.com/bitcoin/bitcoin/pull/17428/commits/c29272a157d09a8125788c1b860e89b63b4cb36c
Depends on D10507
Test Plan: ninja
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Differential Revision: https://reviews.bitcoinabc.org/D10508