Simplify "builtin file" management and recover from races
Just use a dedicated column instead. The transform thing is too clever/weird/hacky and exposes us to issues with the "file" and "transform" tables getting out of sync (possibly the issue in T11307?) and with race conditions.
- Loaded profile "edit picture" page, saw builtins.
- Deleted all builtin files, put 3 second sleep in the storage engine write, loaded profile page in two windows.
- Before patch: one of them failed with a race.
- After patch: both of them loaded.
Reviewed By: chad
Differential Revision: https://secure.phabricator.com/D16271