Merge #11458: Don't process unrequested, low-work blocks

Summary:
01b52ce Add comment explaining forced processing of compact blocks (Suhas Daftuar)
08fd822 qa: add test for minchainwork use in acceptblock (Suhas Daftuar)
ce8cd7a Don't process unrequested, low-work blocks (Suhas Daftuar)

Pull request description:

A peer could try to waste our resources by sending us unrequested blocks with
low work (eg to fill up our disk).  Since e265200 we no longer request blocks until we
know we're on a chain with more than nMinimumChainWork (our anti-DoS
threshold), but we would still process unrequested blocks that had more work
than our tip (which generally has low-work during IBD), even though we may not
yet have found a headers chain with sufficient work.

Fix this and add a test.

Tree-SHA512: 1a4fb0bbd78054b84683f995c8c3194dd44fa914dc351ae4379c7c1a6f83224f609f8b9c2d9dde28741426c6af008ffffea836d21aa31a5ebaa00f8e0f81229e

Backport of Core PR11458
Completes T457

Co-authored-by: Jason B. Cox <contact@jasonbcox.com>

Test Plan: test_runner.py p2p-acceptblock

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1976