Executes command.

            { "op" : "c",
              "ns" : "testdb.$cmd",
              "o" : { "drop" : "fs.files"}
            }

def command(self, ns, raw, **kw):
        """ Executes command.

            { "op" : "c",
              "ns" : "testdb.$cmd",
              "o" : { "drop" : "fs.files"}
            }
        """
        try:
            dbname = raw['ns'].split('.', 1)[0]
            self.dest[dbname].command(raw['o'], check=True)
        except OperationFailure, e:
            logging.warning(e)