Pin the timeline to a given event and paginate all the directions (permalink)

[bnjbvr]

We want to support permalinks in the (UI) timeline of the Rust SDK, that is, the ability to click a link to an event, and navigate around it (in the past and future, i.e. backwards- and forward- paginate).

We do have some support for backwards pagination already in the event cache, so there's some previous work here. Here's how I see the next steps:

• add support for arbitrary pagination in the event cache, backwards- or forward-
  • retrieve the initial data, given an initial event id, using the /context endpoint
  • then allow stateful pagination using a paginator kind of API. Paginating forward in the timeline would return whether we've reached the end of the timeline, at which point the caller may change the focus back to Live.
  • (edge case) handle forged event ids not resolving to an actual event (transform 404 into a proper error)
  • event cache: introduce the Paginator API #3309
• add a "focus" mode to a timeline: either pinned to an event (for permalinks), or live (i.e. listening to sync).
  • make it possible to switch focus
  • (edge case) handle event ids pointing to non-visible (filtered out) events
  • timeline: add event focus mode for permalinks #3329
• (bugfix + nice to have) fix date separator insertion by using a post-processing pass instead of inserting in an ad-hoc manner. This may avoid future headaches when working on forward pagination, or insertion at random spots in the timeline.
  • timeline: insert day dividers as a post-processing phase #3254
  • timeline: Use day divider when updating send state #3288
  • day dividers: don't assume a previous item is at the immediate previous position #3273
  • day divider: don't assume events have event id #3268
  • day divider: make it impossible to handle an event without adjusting day dividers #3267

Part of #3058

Nice to have (deferred)

• (nice to have) when clicking a permalink to an event that's "close by" (TBD), don't switch focus, but prepend events (until the focused event - N events)
• (nice to have) another focus mode that's "latest unread" (= read marker)
• (nice to have) make the paginator stateful object able to read events from the cache before spawning server queries
• (nice to have, but not mandatory for this to happen) reconcile events received from a pagination with cached events.
  • Subsets of the problem to consider: no overlap (easy; throw away/keep in memory until there's some overlap after keeping on paginating), complete overlap (easy; do nothing — maybe replace pagination tokens), partial overlaps (intractable in the worst cases: what if we know A/B/C and receive A/D/C, as there's no way to figure the relative position of B and D — easy if there's linear overlap, aka A/B/C is known and we receive A/B/C/D or D/A/B/C).