Split state groups into a separate data store (#6296)

changelog.d/6245.misc

scripts/synapse_port_db

synapse/config/database.py

synapse/storage/data_stores/__init__.py

synapse/storage/data_stores/main/events.py

...orage/data_stores/main/schema/delta/32/remove_indices.sql

...data_stores/main/schema/full_schemas/54/full.sql.postgres

...e/data_stores/main/schema/full_schemas/54/full.sql.sqlite

synapse/storage/data_stores/main/state.py

synapse/storage/data_stores/state/__init__.py

synapse/storage/data_stores/state/bg_updates.py

...ge/data_stores/state/schema/delta/23/drop_state_index.sql

...torage/data_stores/state/schema/delta/30/state_stream.sql

...ata_stores/state/schema/delta/32/remove_state_indices.sql

...age/data_stores/state/schema/delta/35/add_state_index.sql

synapse/storage/data_stores/state/schema/delta/35/state.sql

...torage/data_stores/state/schema/delta/35/state_dedupe.sql

...rage/data_stores/state/schema/delta/47/state_group_seq.py

...ata_stores/state/schema/delta/56/state_group_room_idx.sql

...storage/data_stores/state/schema/full_schemas/54/full.sql