Only set Safe.J (with mgo.SetSafe) if journaling is enabled

Mongo 2.6 errors if you attempt to set Safe.J when journaling is disabled. We introduce a new function, mongo.JournalEnabled, to detect if journaling is enabled at runtime; we update `state.Open` to use JournalEnabled to decide whether to set Safe.J.
Not totally sure we want to completely drop the check against the actual error?