Update block that's assumed to be valid.

Summary: As per title. This will speedup initial sync.

Test Plan:

rm -rf ~/.bitcoin

And run a full sync

Reviewers: freetrader, sickpig, awemany, kyuupichan, #bitcoin_abc

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D339