Ingest a ThreadView, check several invariants, and produce a list of prev pointers to not-yet-pointed-to messages.

Returns the zero time if there is no recorded last send time (either because the device has never sent a message, or because it hasn't sent one since we started recording).

NewKeyFinder creates a KeyFinder.

If no first query time is found in the local db, this function writes the current time.

All devices are presumed active for the first 24 hours.

FetchRetrier is responsible for tracking any nonblock fetch failures, and retrying them automatically.

KeyFinder remembers results from previous calls to CryptKeys().