1. SyncerState contains a mutex and should never be copied. Honestly, this case
was probably fine but it's just as easy to create a separate snapshot type and
easier to reason about.
2. We need to initialize the syncStates array once at start, before accessing
it. By each syncer state inside each worker, we were racing with calls to
`State()`. Again, this was probably benign, but I don't trust optimizing
compilers.
