Fix a performance issue introduced in Synapse v1.83.0 which meant that purging rooms was very slow and database-intensive. (#15693)

* Add indices required to efficiently validate new foreign key constraints on stream_ordering

* Newsfile

Signed-off-by: Olivier Wilkinson (reivilibre) <oliverw@matrix.org>

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Signed-off-by: Olivier Wilkinson (reivilibre) <oliverw@matrix.org>