Event cache: handle read receipts and unread counts

[bnjbvr]

Now that we have some storage backend for the event cache, we can move the handling of read receipts in there.

Rough list of implementation steps:

• Move support for handling read receipts
  • Identify what the timeline needs
• Move support for unread count too
  • Fuse the "room info" subscriber with the unread count subscriber into a single one (same as in [meta] Latest Event API, the missing one #4112)
• Add function to "mark room as read" using read receipts
• Control where to add read receipts (both for other users, and when marking room as read)

Part of #3058.

[bnjbvr]

Copying some notes from #5723:

The event cache would have the responsibility to track read receipts on events:

• tracking means that the read receipt would move, from one event to another, after we receive new read receipt events via sync
• the event cache would propagate dedicated pings for this kind of tracked receipt updates, so the timeline can use that to move read receipts on its items
• it would offer new APIs to load the tracked receipt on a given event (loading all the receipts of an event is already possible via the state store), so the timeline can still load the initial tracked receipts on each event