Make uploads faster by generating and saving thumbnails in parallel.
We generate each thumbnail in parallel, then send each thumbnail to the
backend image servers in parallel.
Most images have 5 variants: 'preview' (150x150), 180x180, 360x360,
720x720, and 'sample' (850px width). Plus the original file, that's 6
files we have to save. In production we have 2 image servers, so we have
to save each file twice, to 2 remote servers. Doing all this in parallel
should make uploads significantly faster.
Move more of the file-handling logic from UploadService and
StorageManager into MediaAsset. This is part of refactoring posts and
uploads to allow multiple images per post.
