Allow bulk & eager fetching of custom property values

This is a performance improvement. Allow custom properties to have a bulk fetching controller method defined, and to specify if they should be fetched eagerly.

An example:

• native property: authorId

• custom property: author maps authorId to Author instance

• currently the author is lazy loaded every time the property is accessed

• bulk fetching would prefetch the values for the entire page

Additional improvements:

• transitive bulk fetching (fetch all author ids first, then authors)

• bulk fetch the values whenever the first property is accessed (lazy bulk fetching?)

• allow a threadpool to be configured for bulk fetching multiple separate properties