Move DB pool and helper functions into dedicated Database class

scripts/synapse_port_db

synapse/app/_base.py

synapse/app/user_dir.py

synapse/module_api/__init__.py

synapse/storage/_base.py

synapse/storage/background_updates.py

synapse/storage/data_stores/main/__init__.py

synapse/storage/data_stores/main/account_data.py

synapse/storage/data_stores/main/appservice.py

synapse/storage/data_stores/main/cache.py

synapse/storage/data_stores/main/client_ips.py

synapse/storage/data_stores/main/deviceinbox.py

synapse/storage/data_stores/main/devices.py

synapse/storage/data_stores/main/directory.py

synapse/storage/data_stores/main/e2e_room_keys.py

synapse/storage/data_stores/main/end_to_end_keys.py

synapse/storage/data_stores/main/event_federation.py

synapse/storage/data_stores/main/event_push_actions.py

synapse/storage/data_stores/main/events.py

synapse/storage/data_stores/main/events_bg_updates.py