Properly split the FileStore into read-only and r/w variants

The ReadOnlyFileStore class currently simply overrides the FileStore class replacing all mutator methods with a trivial implementation. This approach however leaks into its ancestor as the read only store needs to pass a flag to the constructor of its super class so some fields can be instantiated properly for the read only case.

We should clean this up to properly separate the read only and the r/w store. Most likely we should factor the commonalities into a common, abstract base class.