Add `limit` to `get_current_hosts_in_room`

[matrixbot]

This issue has been migrated from #13659.

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Follow-up to matrix-org/synapse#13575. Part of matrix-org/synapse#13356

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To improve things further we could add a limit parameter to get_current_hosts_in_room. Realistically, we don't need 4k domains to choose from because there is no way we're going to query that many before we a) probably get an answer or b) we give up.

-- Going further section in matrix-org/synapse#13575

This means we can do less work in the database which means less waiting to get a list of domains to try.