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/Cache.hs

@@ -1,4 +1,4 @@
-module Cache (Cache,newCache,readCache) where
+module Cache (Cache,newCache,flushCache,readCache) where
 
 import Control.Concurrent.MVar
 import Data.Map (Map)
@@ -17,6 +17,9 @@ newCache load =
     do objs <- newMVar Map.empty
        return $ Cache { cacheLoad = load, cacheObjects = objs }
 
+flushCache :: Cache a -> IO ()
+flushCache c = modifyMVar_ (cacheObjects c) (const (return Map.empty))
+
 readCache :: Cache a -> FilePath -> IO a
 readCache c file = 
     do v <- modifyMVar (cacheObjects c) findEntry