R1:2b0b9a15734c

Add a generic multistep Markup cache

Summary:
The immediate issue this addresses is T1366, adding a rendering cache to Phriction. For wiki pages with code blocks especially, rerendering them each time is expensive.

The broader issue is that out markup caches aren't very good right now. They have three major problems:

**Problem 1: the data is stored in the wrong place.** We currently store remarkup caches on objects. This means we're always loading it and passing it around even when we don't need it…