[hooks/syncschema] only call "ALTER TABLE" once when changing a size constraint Until now we would: - remove the old size constraint from the in-memory schema - call update_rdef_column which removes the size restriction from the column's type - add the new constraint object - call update_rdef_column which adds the size restriction back This breaks on SQL Server when the column is involved in an index (e.g. as part of a multi-column unique constraint), because in the intermediate stage the column's type is "nvarchar(max)", which is not indexable. Of course we must still detect the case where a size constraint is really dropped and update the db schema accordingly. Closes #5557633.

[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.