Merge PDUs and Events into one object

synapse/api/events/__init__.py

synapse/api/events/utils.py

synapse/crypto/event_signing.py

synapse/federation/pdu_codec.py

synapse/federation/replication.py

synapse/federation/units.py

synapse/handlers/federation.py

synapse/storage/_base.py

tests/federation/test_federation.py

tests/handlers/test_federation.py