[net processing] pass a Peer to FindNextBlocksToDownload

Summary:
This change was skipped when backporting core#25514 because the rest of that commit is related to segwit and does not apply to Bitcoin ABC. We could just keep ignoring the change but then we have to work around the difference in function signature in future backports (e.g. core#27596), so I think it is simpler to closely follow Core on this one.

This is a partial backport of https://github.com/bitcoin/bitcoin/pull/25514/commits/7d1c0369340cb752f0d78e24f4251595534bf5e9

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, roqqit

Reviewed By: #bitcoin_abc, roqqit

Differential Revision: https://reviews.bitcoinabc.org/D17631