SYN-383: Extract the response list from 'server_keys' in the response JSON as it might work better than iterating over the top level dict

Notifier performance

Move get_events functions to storage.events

Make sure the notifier stream token goes forward when it is updated. Sort the pending events by the correct room_stream_id

Add some doc strings for presence.

Make v1 and v2 client APIs interact with the notifier in the same way.

Use the current token when timing out a notifier, make sure the user_id is a string in on_new_user_event

Don't bother checking for new events from a source if the stream token hasn't advanced for that source

Conflicts:
	synapse/notifier.py

Revert "Improvement to performance of presence event stream handling"