fix(#3277844): stage each upload in a directory of its own

Stages each upload in a directory of its own under the temporary file location, so two uploads of a file with the same name never share a staged path or an image style preview URL, and removes that directory once empty when cron deletes an upload that was never saved. A site with no staging location configured, or one set to a bare scheme root, stages under that root. Kernel, unit and functional tests included.