[syncschema] only add to the `synchronized` set if all possible updates have been done
Otherwise a simple sync_schema(syncprops=False) turns the subsequent
sync_schema() calls into a no-op.
Closes #6510918.
Schema of the data
------------------
First take a look at the data schema_ then try to remember that you are browsing
through a heap of data by applying stylesheets to the results of queries. This
site is not a content management system with items placed in folders. It is an
interface to a database which applies a view to retreived data.
.. _schema: ../schema