Allocate notify-related state lazily instead. This makes trivial
subtransactions noticeably faster.

Patch by me, reviewed and tested by Dilip Kumar, Kyotaro Horiguchi,
and Jeevan Ladhe.

Discussion: https://postgr.es/m/CA+TgmobE1J22S1eC-6N-je9LgrcwZypkwp+zH6JXo9mc=4Nk3A@mail.gmail.com