Use in-memory SQLite

[penberg]

We currently use file-backed SQLite databases because the basic in-memory SQLite does not seem to support concurrent reads:

https://github.com/chiselstrike/chiselstore/commit/75ec0c75950499a91ef5dffcdc368cdc67924e38

Let's look at ways to turn on in-memory again. For example, the "memdb" VFS should support concurrent reads.