[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.
usr/lib/PY_VERSION/*-packages/cubicweb/server/
usr/lib/PY_VERSION/*-packages/cubicweb/hooks/
usr/lib/PY_VERSION/*-packages/cubicweb/sobjects/
usr/lib/PY_VERSION/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/