High DB usage on pusher processes

Sometimes our pusher workers spend a period of time hammering the database via `get_unread_counts_by_room_for_user`.

I believe https://github.com/element-hq/synapse/pull/16756 was an attempt to improve it, but that got reverted in https://github.com/element-hq/synapse/pull/16979.

Per https://github.com/matrix-org/matrix-spec-proposals/pull/4076, we should give apps a way to opt-out of receiving the badge counts, because it's usually wrong and isn't necessary.

Per https://github.com/matrix-org/matrix-spec/issues/644, what we return isn't correct according to the spec, anyway.