Use a single round of SHA256 ont he CHECKDATASIG message.
Summary: As per suggestion in the spec review.
Test Plan: Updated test cases.
Reviewers: #bitcoin_abc, jasonbcox, Mengerian
Reviewed By: #bitcoin_abc, jasonbcox, Mengerian
Subscribers: Mengerian, awemany, jasonbcox, teamcity
Differential Revision: https://reviews.bitcoinabc.org/D1653