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