Fix sliding sync performance slow down for long lived connections. (#19206)

Fixes https://github.com/element-hq/synapse/issues/19175

This PR moves tracking of what lazy loaded membership we've sent to each
room out of the required state table. This avoids that table from
continuously growing, which massively helps performance as we pull out
all matching rows for the connection when we receive a request.

The new table is only read when we have data in a room to send, so we
end up reading a lot fewer rows from the DB. Though we now read from
that table for every room we have events to return in, rather than once
at the start of the request.

For an explanation of how the new table works, see the
[comment](https://github.com/element-hq/synapse/blob/erikj/sss_better_membership_storage2/synapse/storage/schema/main/delta/93/02_sliding_sync_members.sql#L15-L38)
on the table schema.

The table is designed so that we can later prune old entries if we wish,
but that is not implemented in this PR.

Reviewable commit-by-commit.

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Co-authored-by: Eric Eastwood <erice@element.io>

changelog.d/19206.bugfix

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+Fix sliding sync performance slow down for long lived connections.