Resolve timezone conflicts in a modern transactional way

Summary: Ref T4103. Also get rid of the weird cache clear that nothing else uses and which we don't actually need.

Test Plan:

• Resolved timezone conflict by ignoring it.
• Resolved timezone conflict by picking a valid timezone.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4103

Differential Revision: https://secure.phabricator.com/D16037