Prevent ambiguous URIs

[ScottDugas]

When specifying a URI (i.e. COPY /my/database/path), it could be ambiguous depending on the underlying KeySpace. Particularly, imagine the situation where you have:

KeySpace(new KeySpaceDirectory("root", STRING, "R"),
    new DirectoryLayerDirectory("otherData"))

This is an unambiguous path, because otherData represents as a long on disk, and the name is included when creating the path. But, when represented as a path, it is unclear what /R points to.
When parsing the path, we should ensure that it is unambiguous, and could not be parsed as a different path.

We should try to minimize the set of invalid KeySpace structures, to maximize backwards compatibility with existing KeySpace structures.