Asset cache can lead to denial of service if asset database is deleted

[echlebek]

Expected Behavior

When the asset database is deleted, but the asset cache is not, the system should still work as expected.

Current Behavior

When the asset database is deleted, but the asset cache is not deleted, then the asset downloader encounters errors about files already existing when trying to populate the cache.

Possible Solution

Replace existing files when the database is missing an entry.