Batch look-ups to see if rooms are partial stated. (#14917)

* Batch look-ups to see if rooms are partial stated.

* Fix issues found in linting.

* Fix typo.

* Apply suggestions from code review

Co-authored-by: Sean Quah <8349537+squahtx@users.noreply.github.com>

* Clarify comments.

Co-authored-by: Sean Quah <8349537+squahtx@users.noreply.github.com>

* Also improve the cache size while we're at it

* is_partial_state_rooms -> is_partial_state_room_batched

* Run `black`

* Improve annotation for `simple_select_many_batch`

* Fix is_partial_state_room_batched impl

* Okay, _actually_ fix impl

* Update description.

* Update synapse/storage/databases/main/room.py

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

* Run black.

Co-authored-by: Sean Quah <8349537+squahtx@users.noreply.github.com>
Co-authored-by: David Robertson <davidr@element.io>