Implement Prefer: HTTP headers to control LDPC hierarchy serialization

* Implement Prefer: HTTP headers to control LDPC hierarchy serialization
* Implement LDP Paging Spec

Fixes https://www.pivotaltracker.com/story/show/69467742 and https://www.pivotaltracker.com/story/show/69469628
https://www.pivotaltracker.com/story/show/69633558