Now that we don't expect there to be duplication betwen the 'timeline' and
'state' results, there's no point in having the state map. (That does mean the
events themselves need event_id fields though).
Also:
- move the contents of the 'unsigned' dictionary into the events themselves
- replace the state list with two layers of dictionary keyed on type and
state_key
- rename the children of the 'rooms' dict from "joined/invited/archived" to
"join/invite/leave" to match the membership states
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