PGFService: make the restriction on parallelism more effective

Restrict the entire request handler instead of just the call to the C parser.

src/server/gf-server.cabal

@@ -38,6 +38,7 @@ Library
                    cgi >= 3001.1.7.3,
                    httpd-shed>=0.4.0.2,
                    mtl,
+                   exceptions,
                    network>=2.3 && <2.6,
                    json >= 0.3.3,
                    utf8-string >= 0.3.1.1,