PGF Service: limit the number of parallel calls to the C run-time parse function to 4 by default

The limit can be changed with the -j flag

src/compiler/GF/Interactive.hs

@@ -71,11 +71,12 @@ shell opts files = loop opts =<< runSIO (importInEnv emptyGFEnv opts files)
 -- | Run the GF Server (@gf -server@).
 -- The 'Int' argument is the port number for the HTTP service.
 mainServerGFI opts0 port files =
-    server port root (execute1 opts)
+    server jobs port root (execute1 opts)
       =<< runSIO (importInEnv emptyGFEnv opts files)
   where
     root = flag optDocumentRoot opts
     opts = beQuiet opts0
+    jobs = join (flag optJobs opts)
 #else
 mainServerGFI opts files =
   error "GF has not been compiled with server mode support"