Sliding Sync: Speed up background updates to populate Sliding Sync tables

[MadLittleMods]

Speed up background updates to populate Sliding Sync tables

Follow-up to https://github.com/element-hq/synapse/pull/17512

• Read from the room_stats_state table for room_type, name, and is_encrypted
• For joins, if the state hasn't changed since the last snapshot, use the last membership snapshot we inserted
• concurrently_execute processing and inserting
• Use batch/bulk SQL queries like simple_upsert_many_txn where we can

Dev notes

The quick and dirty script that Erik wrote to run on matrix.org, https://github.com/erikjohnston/synapse-fast-bg-update/blob/main/src/main.rs

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