add clock skew compensation again

this has the negative effect of causing multiple full syncs if syncing
multiple times within a 5 minute period of the previous full sync, but it
makes it much less likely that people's due counts will fall out of sync

anki/sync.py

@@ -122,7 +122,7 @@ class SyncTools(object):
         l = self._lastSync(); r = self.server._lastSync()
         # set lastSync to the lower of the two sides
         if l != r:
-            self.deck.lastSync = min(l, r)
+            self.deck.lastSync = min(l, r) - 350
         else:
             self.deck.lastSync = l
         return True