[SCHEMA] EAS Two-Device-Per-Account migrations.

Also,
Add BaseMailSyncMonitor._cleanup()
Move thread state fns out of BaseMailSyncMonitor.
Update bin/clear-db.

And,
Migration to fix EASUid cascade deletes.

Summary: Matching changes to D629.

Test Plan: None

Reviewers: emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D632