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.
Most PGF web API commands that produce linearizations now accept an
unlexer parameter. Possible values are "text", "code" and "mixed".
The web service now include Date and Last-Modified headers in the HTTP,
responses. This means that browsers can treat responses as static content and
cache them, so it becomes less critical to cache parse results in the server.
Also did some cleanup in PGFService.hs, e.g. removed a couple of functions
that can now be imported from PGF.Lexing instead.
