Define semantics for state when filtered (SPEC-337)

What should happen if a user does a `/sync`, with a filter that excludes state from the timeline, but includes it in the state section?

Before, the "state" key included the state at the _end_ of the timeline, now it is the state at the _start_ of the timeline. This is problematic due to the fact you won't see any of the state updates in the timeline section (if filtered in timeline), and so the client can't work out the current state.

(Imported from https://matrix.org/jira/browse/SPEC-337)

(Reported by @erikjohnston)

Start with the /sync behavior described in this issue, focusing on how filters affect the timeline and state sections. Determine and document the agreed semantics for state when state updates are excluded from the timeline; the issue is complete when the behavior is explicitly specified.

Written by the indexing model from the issue text.