Cached AssetStore-index

[rawler]

The contents of the assetstore should preferably be cached in RAM, to avoid excessive disk-I/O.

[rawler]

This is partially solved by b726af7, but we can do better.

Caching is a delicate problem, with lots of tuning. Fortunately, there are those who have already done it.

The long-term plan is to replace all per-hash symlinks with a small SQLite-database. This will reduce inodes and load on the filesystem, come with free RAM-caching, and allow easily storing some extra metadata about the assets, such as completion-levels, and correctly used, thread-safety allowing background scans and updates.