Bust `_membership_stream_cache` cache when current state changes

[MadLittleMods]

Spawning from #17320 (comment)

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Bust _membership_stream_cache cache when current state changes.

Currently, the only reference I can find is in CacheInvalidationWorkerStore._invalidate_caches_for_event(...) which doesn't handle current state changing.

Dev notes

This is where we should be adding the cache busting:

synapse/synapse/storage/databases/main/cache.py

Line 266 in 956f20e

elif row.type == EventsStreamCurrentStateRow.TypeId: