Add functionality to flush the internal cache

Before we had the problem that when iterating over _a lot_ (like 5k)
entries and also fetching them, at some point the OS would return with
"Too many files open".

That is because the store internally caches a lot.

With this change, the Store gets an API to query how big the cache is,
how much the cache can currently hold and (and that's the main thing in
this patch) to flush the cache to disk.

A function to simply ask the store whether its cache should be flushed
(which would us require to ask the OS how many files we can open...
which would be possible with `libc::getrlimit`) does not yet exist,
though, but could be added easily if desired.

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