No more silent loss during sync

Fixed

• Sync no longer moves past an event it cannot read. An event of a type unknown to this machine was dropped silently, then the cursor advanced over it: the data ended up permanently behind, with no way back to it. Such events are now recorded along with the resume point needed to replay them once the machine is up to date.
• A page made up entirely of unknown types did not advance at all, blocking sync for the types that did work.