Increase DB/CPU perf of `_is_server_still_joined` check. (#6936)

* Increase DB/CPU perf of `_is_server_still_joined` check.

For rooms with large amount of state a single user leaving could cause
us to go and load a lot of membership events and then pull out
membership state in a large number of batches.

* Newsfile

* Update synapse/storage/persist_events.py

Co-Authored-By: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* Fix adding if too soon

* Update docstring

* Review comments

* Woops typo

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>