This changes how files work. There is now an open function that
translates a path to a file handle (id). Then this handle is used
to perform other actions on the file. The file contents are moved
to a separate table so that the actual file storage is independent
of the database (e.g. in the future, we may use File Access API).
We store a hash of the file contents in the metadata file so we can
detect file changes without having to compare file contents.
We also separate the change and add actions.
