Threads: /messages filter to exclude thread responses & relations

**Is your feature request related to a problem? Please describe.**
Right now, scrolling up in a room which heavily uses threads is really slow and feels broken as most /messages results are not for the main timeline and instead are events from threads.

**Describe the solution you'd like**
/messages filter which lets the client paginate the main timeline where the responses only include events for that timeline

**Describe alternatives you've considered**
Making the server go faster so this is less painful but it is still quite wasteful of data and worst case could still result in many pages of content which cannot be rendered in that timeline.

Start by reviewing the /messages endpoint and how thread responses and relations are currently included in its results. Define the filter semantics and pagination behavior for returning only events belonging to the main timeline; done means clients can paginate that timeline without receiving unrelated thread events.

Written by the indexing model from the issue text.