synapse tracks lazy_loaded_members_cache per user/device but it should be per stream of events

This issue has been migrated from [#5421](https://github.com/matrix-org/synapse/issues/5421).

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i.e. if we stripped redundant events in /messages, it should use a separate cache to /sync. whereas the current code assumes they share a cache

Search for lazy_loaded_members_cache and trace how it is used by the /messages and /sync event streams. Confirm whether redundant events are stripped through shared state, then separate the cache state per stream and verify that each stream tracks its own lazy-loaded members independently.

Written by the indexing model from the issue text.