Datastore: read-only cache that uses a key-value store

For data stores that are large and contain a lot of small entries, the overhead of the current file cache is quite large: at least one file per entry is needed. For most file systems, that means 4 KB per entry. If entries are much smaller (e.g. 30 bytes), the overhead is large.

We could instead use a read-only key-value store for such entries.