Fixes to sync()

* Save/sync executor attempted to commit changes to pydantic objects
  even when there was an exception inside its context manager

* Sync was writing changes to pydantic objects while the transaction was
  still open

* Fix sync() to update single links properly and handle the case when a
  linked object is replaced with a different one

* Drop BlockingClient.__save_debug__ method, it's quite outdated and
  must be broken