Caching parse results uses a lot of memory, even if they expire after
2 minutes, so it won't scale up to many simultaneous users.
But some excessive memory use seems to be caused by space leaks in
(the Haskell binding to) the C run-time system, and these should be fixed.
For example, flushing the PGF cache does not release the memory allocated
by the C run-time system when loading a PGF file.