Merges context from this instance with src.

def merge_context(self, src=None):
        """Merges context from this instance with src."""
        if src is None:
            # This condition will be true only if the server doesn't return a
            # txn context after a query or mutation.
            return

        if self._ctx.start_ts == 0:
            self._ctx.start_ts = src.start_ts
        elif self._ctx.start_ts != src.start_ts:
            # This condition should never be true.
            raise Exception('StartTs mismatch')

        self._ctx.keys.extend(src.keys)
        self._ctx.preds.extend(src.preds)