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.