R1:af06bfb1cc49

Make Changeset ID for render cache explicit

Summary:
DifferentialChangesetParser currently takes the Changeset object to mean a bunch
of different and mutually conflicting things implicitly:

- Changeset ID is used to access the render cache.
- Changeset ID is also used to tell the ajax endpoint what to render when
clicking "show more".
- Changeset object has the actual changes.
- Changeset ID and "oldChangesetID" are used to choose where to show inline
comments and how to attach new ones.

This…