Add support for resolving references to nodes stored in the file system connector

Enhance the filesystem connector so that it optionally supports referenceable nodes. If this option is enabled, the nodes would be of an extended type that includes mix:referenceable. Per the JCR spec, I believe this means that the identifiers for these nodes must be immutable. For example, when a node is first accessed, a UUID couuld be generated. It and the nodes current path could be stored in the query (Lucene) index, and vice versa. Whenever a node is moved or updated, that index entries would be updated. Thus, a mapping from unique immutable UUID and current path would exist, supporting immutable identifiers, and file lookup given the identifier.

Enabling this feature implicitly requires the underlying files to never be moved or deleted directly on the file system level, obviously since this will invalidate the UUID <--> Path mapping. However this seems to be a reasonable. If that cannot be enforced, then the option for referenceable nodes cannot be used.

Please see the forum post for more info.