Fixed the error "cannot pickle sqlite3.Cursor"

[micheles]

It happened only with cache=true in the lines

                logging.info(f"Already calculated, {old_job_id=}")
                self.datastore = datastore.read(old_job_id)
                logs.dbcmd(
                    "UPDATE job SET ds_calc_dir = ?x WHERE id=?x",
                    self.datastore.filename[:-5], calc_id)  # strip .hdf5

because in presence of a DbServer the update query returns a sqlite3.Cursor object which is not pickleable.
Also, if the database is locked, just don't log; we do not want to break the calculation in DbServer mode (we still want to break it with a local db since errors should never pass silently).