Should avoid side affects from namespace reuse

[otherjoel]

Pollen no longer explicitly resets global variables for each page render. The previous behaviour was never explicitly guaranteed, and now it is explicitly not guaranteed. Our citations implementation depends on mutating global variables, and is therefore currently broken!

There are several ways to solve this, but the simplest solution in this case will be to store our citations hash table in current-metas, which is kept fresh for each page.