Cancellation takes a long time to finish

[adamhooper]

Steps to reproduce:

1. Alice uploads a big set of PDFs
2. Bob uploads a small set of PDFs
3. Bob cancels the conversion

Expected results: the conversion cancels quickly
Actual results: the cancellation is stalled while waiting for Alice's documents to convert

This affected a user today.

[adamhooper]

An enormous slowdown is that cancellation requires deleting all uploaded Large Objects -- and Large Objects are slow. Let's make users upload to S3 directly, where deletes are way faster (and disk space is ample).