The postgres statistics collector sometimes massively underestimates the
number of distinct state groups are in the `state_groups_state`, which
can cause postgres to use table scans for queries for multiple state
groups.
We fix this by manually setting `n_distinct` on the column.
The hope here is that by moving all the schema files into synapse/storage/schema, it gets a bit easier for newcomers to navigate.
It certainly got easier for me to write a helpful README. There's more to do on that front, but I'll follow up with other PRs for that.