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

NewAttachmentStore creates a standard AttachmentStore that uses a real S3 connection.

NewKeyFinder creates a KeyFinder.

Preview creates preview assets from src.

ErrAbortOnPartMismatch is returned when there is a mismatch between a current part and a previous attempt part.

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

PutS3Result is the success result of calling PutS3.

KeyFinder remembers results from previous calls to CryptKeys().