Change the way pagination works to support out of order events.

synapse/api/streams/__init__.py

synapse/api/streams/event.py

synapse/handlers/presence.py

synapse/handlers/room.py

synapse/storage/schema/im.sql

synapse/storage/stream.py

webclient/components/matrix/event-stream-service.js

webclient/components/matrix/matrix-service.js