Accept unrequested blocks with work equal to our tip

Summary:
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)
https://github.com/bitcoin/bitcoin/pull/11531/commits/932f118e6a3779bb3d6c3cc83963cf34ac150e42

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