Sliding sync: Store the per-connection state in the database. (#17599)

changelog.d/17599.misc

synapse/app/generic_worker.py

synapse/handlers/sliding_sync/__init__.py

synapse/handlers/sliding_sync/store.py

synapse/storage/database.py

synapse/storage/databases/main/__init__.py

synapse/storage/databases/main/sliding_sync.py

synapse/storage/engines/_base.py

synapse/storage/engines/postgres.py

synapse/storage/engines/sqlite.py

synapse/storage/schema/__init__.py

.../storage/schema/main/delta/87/02_per_connection_state.sql

synapse/types/handlers/sliding_sync.py

tests/rest/client/sliding_sync/test_rooms_required_state.py