Deduplicate concurrent requests to get_state_for_groups

[reivilibre]

Closes #10510. Closes #10301.

[reivilibre]

This is a draft reincarnation of #10510.

I don't fully like it. It feels like it should be possible to do something simpler, but there are quite a few cases to consider (mostly due to the include_others flag on StateFilters). That said, I haven't come up with anything I like more so far.

I will write tests for this, because I feel like it really needs them to exercise all the conditions, but I was hoping to ask for advice on this before I sink the effort to write some tests in (though maybe that's not such a concern since they might be reusable?).

[erikjohnston]

I think @reivilibre was going to try and tidy up the commits a bit and then re-request review

[erikjohnston]

@reivilibre and I had a chat about this and we're changing the way we cache things a bit to avoid having a new type of cache.