write-back cache

After the initial implementation of #166 in #168 got some testing, we could also add a write-back cache.

For tools like borgbackup, doing an async write-back cache could speed up backups a lot:

A ``Store.store`` operation would return after cache has been updated, but would not wait until slower primary backend got updated.

But that also comes with some complexity:
- guess we'll need some sort of ``sync`` or write barriers to keep primary backend updates in correct order (users usually will want to have data items written before reference to them from metadata items)
- multi-threading or async
- potential errors with the primary backend will happen async