Clear our pool for a server and mark it Unknown.

        Hold the lock when calling this. Does *not* request an immediate check.

def _reset_server(self, address):
        """Clear our pool for a server and mark it Unknown.

        Hold the lock when calling this. Does *not* request an immediate check.
        """
        server = self._servers.get(address)

        # "server" is None if another thread removed it from the topology.
        if server:
            server.reset()

            # Mark this server Unknown.
            self._description = self._description.reset_server(address)
            self._update_servers()