Fallback logic is moved to the caller's responsibility.
Empty FileStorages and FileStorages that load from the server are
separated into different classes. This is to avoid faults where the
caller of the constructor forgets to pass in a `baseurl` parameter and
leads to some confusing bug.
Uses MemoryFileStorage as a fallback when:
- v86 is used in a browserless environment, e.g. NodeJS
- browser doesn't support indexedDB
- opening indexedDB fails
- existing database is a newer version
- existing database is an old version and is under use - blocking
current v86 instance to connect to it.
