Accept unrequested blocks with work equal to our tip
This is a simple change that makes our accept requirements the
same as our request requirements, (ever so slightly) further
decoupling our consensus logic from our FindNextBlocksToDownload
logic in net_processing.
Partial backport of Core PR 11531 (932f118)
This looks safe to me since it does not introduce any DoS vector beyond what currently exists.
Test Plan: test_runner.py --extended
Reviewers: deadalnix, Fabien, #bitcoin_abc
Reviewed By: deadalnix, Fabien, #bitcoin_abc
Subscribers: teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2254