This addresses atomicity of tag synchronization, the last atomicity
problems in notmuch new. Each message add or remove is wrapped in its
own atomic section, so interrupting notmuch new doesn't lose progress.
This tests notmuch new's ability to recover from arbitrary stopping
failures. It interrupts notmuch new after every database commit and,
on every resulting database snapshot, re-runs notmuch new to
completion and checks that the final database state is invariant.
