Make Downloader download to a tmp dir, then move it to cache on success

Downloader currently d/ls straight to the cache directory. Partial downloads caused by a crash, or who knows what, could make Cache return that partial data and mess everything up. To fix that, Downloader should d/l to a tmp dir (ex, storagemanager/tmp), then move it to the cache after success.

This would also apply to Synchronizer. When it merges a journal with its object, the merged object is created directly in the cache dir.