`Store`: Support more options for `open()`

[clarisma]

Store (and its progeny, including FeatureStore) should allow more control over what happens in open()

• create a new store file if one does not exist (current behavior), or throw FileNotFoundException
• read-only vs. read/write (current behavior)

These options should be combined with the one currently supported option (shared or exclusive lock).

This enables support for #87 (An empty GOL file should only be created if a tileset URL is specified).