move DeferredCache into its own module

synapse/replication/slave/storage/client_ips.py

synapse/storage/databases/main/client_ips.py

synapse/storage/databases/main/devices.py

synapse/storage/databases/main/events_worker.py

synapse/util/caches/deferred_cache.py

synapse/util/caches/descriptors.py

tests/storage/test__base.py

tests/test_metrics.py

tests/util/caches/test_deferred_cache.py

tests/util/caches/test_descriptors.py