ysync: DefaultProtocol - after doc update, reapply step 1 if pending update was found

[Horusiath]

In general when we get out-of-order or missing update, we should try to restart connection and start from step 1 (sending state vectors). However it's not always easy to detect. We check if after update application, there are still pending updates and depending on that:

1. Trigger an Err in order to signal missing update. This may be bad if missing update was on the way (however the rule that updates still should arrive in order should apply).
2. Return SyncStep1 reply that will be send back to the requester.

Eventually we can split that into two, and provide two protocol implementation - one triggering restart via pt.1 another sending sync step back.