Speed up fetching partial-state rooms on sliding sync (#17666)

Instead of having a large cache of `room_id -> bool` about whether a
room is partially stated, replace with a "fetch rooms the user is which
are partially-stated". This is a lot faster as the set of partially
stated rooms at any point across the whole server is small, and so such
a query is fast.

The main issue with the bulk cache lookup is the CPU time looking all
the rooms up in the cache.