Merge pull request #697 from matrix-org/markjh/slaveI

synapse/replication/slave/__init__.py

synapse/replication/slave/storage/__init__.py

synapse/replication/slave/storage/_base.py

synapse/replication/slave/storage/_slaved_id_tracker.py

synapse/replication/slave/storage/events.py

synapse/storage/events.py

tests/replication/slave/__init__.py

tests/replication/slave/storage/__init__.py

tests/replication/slave/storage/_base.py

tests/replication/slave/storage/test_events.py