PGFService: make the restriction on parallelism more effective

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

src/server/exec/pgf-fcgi.hs

@@ -5,7 +5,7 @@ import Network.FastCGI(runFastCGI,runFastCGIConcurrent')
 import PGFService(cgiMain,newPGFCache,stderrToFile,logFile)
 
 main = do stderrToFile logFile
-          fcgiMain =<< newPGFCache
+          fcgiMain =<< newPGFCache Nothing
 
 
 fcgiMain cache =