R1:b7fa55ff9396

Fix improper selection of the chunk engine as a writable engine

Summary:
Fixes T7621. The engine selection code started out making sense, but didn't make as much sense by the time I was done with it.

Specifically, from the vanilla file upload, we may incorrectly try to write directly to the chunk storage engine. This is incorrect, and produces a confusing/bad error.

Make chunk storage engines explicit and don't try to do single-file one-shot writes to them.

Test Plan:
- Tried to upload a large file…