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 indirectly causes a bunch of problems, like T141 and T132. Move toward
making all these separate things explicit. I want to have the changeset object
only mean the actual changes to display.

Test Plan:
Looked at changesets and verified the render cache was accessed correctly (and
not accessed in other cases).

Reviewed By: tuomaspelkonen
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, epriestley, tuomaspelkonen
Differential Revision: 228