A0-4006: Check initial unit control hashes early

[timorleph]

Control hashes of initial units have to refer to the empty parent set, so their combined hash is actually known a priori and can be checked early. This also changes one test that used to assume a initial unit with a broken hash can be passed into consensus.

The only connection to A0-4006 is that the test in question failed after the refactor, since I didn't want to react to this incongruous situation internally.

[github-actions[bot]]

Please make sure the following happened

• [ ] Appropriate tests created
• [ ] Infrastructure updated accordingly
• [ ] Updated existing documentation
• [ ] New documentation created
• [ ] Version bumped if breaking changes