net_processing: Request assumeutxo background chain blocks

Summary:
Add new PeerManagerImpl::TryDownloadingHistoricalBlocks method and use it to
request background chain blocks in addition to blocks normally requested by
FindNextBlocksToDownload.

Co-authored-by: Ryan Ofsky <ryan@ofsky.org>
Co-authored-by: James O'Beirne <james.obeirne@gmail.com>

This is a partial backport of core#27596
https://github.com/bitcoin/bitcoin/pull/27596/commits/b73d3bbd23220857bf17cbb6401275bf58013b72

Depends on D17631

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, roqqit

Reviewed By: #bitcoin_abc, roqqit

Subscribers: roqqit

Differential Revision: https://reviews.bitcoinabc.org/D17632