gf -server: add a command to manually flush the PGF cache

This can be used if the cloud service seems slow, but it would probably
be better to automatically expire unused PGFs from the cache after some time.

src/server/PGFService.hs

@@ -1,7 +1,7 @@
 {-# LANGUAGE CPP #-}
 module PGFService(cgiMain,cgiMain',getPath,
                   logFile,stderrToFile,
-                  newPGFCache) where
+                  newPGFCache,flushPGFCache) where
 
 import PGF (PGF)
 import qualified PGF
@@ -51,10 +51,12 @@ newPGFCache = do pgfCache <- newCache PGF.readPGF
                                                    pc <- newMVar Map.empty
                                                    return (pgf,pc)
                  return (pgfCache,cCache)
+flushPGFCache (c1,c2) = flushCache c1 >> flushCache c2
 #else
 type Caches = (Cache PGF,())
 newPGFCache = do pgfCache <- newCache PGF.readPGF
                  return (pgfCache,())
+flushPGFCache (c1,_) = flushCache c1
 #endif
 
 getPath =