Add a checkpoint at block height 680140

Summary:
This will make it easier to sync the node in the event the alternative
chain resumes.

Test Plan: Run IBD.

Reviewers: #bitcoin_abc, majcosta, Mengerian

Reviewed By: #bitcoin_abc, majcosta, Mengerian

Subscribers: Mengerian, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9367