Room: fix the read markers/receipts confusion

This turns the design changes laid out in #464 comments to code:
- readMarker() now returns the fully read marker, unlike
  readMarker(User*) that returns a read receipt, even when called for
  the local user.
- Private::setLastReadEvent() -> setLastReadReceipt(), incorporating
  the "promotion" logic from promoteReadReceipt().
- The above makes promoteReadReceipt() unneeded; the remaining piece
  of logic that recalculates the number of unread messages is put to
  its own method - Private::recalculateUnreadCount().
- Private::updateUnreadCount() is only slightly refreshed, continues
  to use the fully read marker position (as it used to).
- Now that read receipts and fully read markers are managed separately,
  Private::setLastReadReceipt() has got its counterpart,
  Private::setFullyReadMarker(); both only update their respective
  markers locally (emitting signals as needed), without interaction
  with the homeserver.
- Private::markMessagesAsRead() now delegates updating the fully read
  marker to setFullyReadMarker() and on top of that sends the new
  fully read marker to the homeserver.
- Private::serverReadMarker -> fullyReadUntilEventId (to be really clear
  what it stores).
- The hand-written PostReadMarkersJob is replaced with the generated
  SetReadMarkerJob that does the same thing (and can update the read
  receipt on top, though the current code doesn't use that).

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