SYN-75 Verify signatures on server to server transactions

synapse/crypto/keyclient.py

synapse/crypto/keyring.py

synapse/crypto/keyserver.py

synapse/crypto/resource/__init__.py

synapse/federation/replication.py

synapse/federation/transport.py

synapse/federation/units.py

synapse/server.py

synapse/storage/__init__.py

synapse/storage/keys.py

synapse/storage/schema/keys.sql

tests/federation/test_federation.py

tests/handlers/test_presence.py

tests/handlers/test_typing.py