Unlock a previously locked server.

def unlock(self):
        """Unlock a previously locked server.
        """
        cmd = {"fsyncUnlock": 1}
        with self._socket_for_writes() as sock_info:
            if sock_info.max_wire_version >= 4:
                try:
                    sock_info.command("admin", cmd)
                except OperationFailure as exc:
                    # Ignore "DB not locked" to replicate old behavior
                    if exc.code != 125:
                        raise
            else:
                helpers._first_batch(sock_info, "admin", "$cmd.sys.unlock",
                    {}, -1, True, self.codec_options,
                    ReadPreference.PRIMARY, cmd, self._event_listeners)