Fix performance of check_state_groups_and_bump_deletion

[erikjohnston]

Regressed as part of #18107

This does two things:

1. Only check if the state groups have been deleted when calculating the event context (as that's when we will insert them). This avoids lots of checks for read operations.
2. Don't lock the state_groups rows when doing the check. This adds overhead, and it doesn't prevent any races.

[devonh]

The code looks clean. Added comments are nice.
The logic changes make sense to me. But I also approved the previous PRs that got us into this state in the first place... :)