Allow parallelism for deferrable serializable txns

Hi,

The code currently says:

     * We can't use parallelism in serializable mode because the predicate
     * locking code is not parallel-aware.  It's not catastrophic if someone
     * tries to run a parallel plan in serializable mode; it just won't get
     * any workers and will run serially.  But it seems like a good heuristic
     * to assume that the same serialization level will be in effect at plan
     * time and execution time, so don't generate a parallel plan if we're in
     * serializable mode.
     */
    if ((cursorOptions & CURSOR_OPT_PARALLEL_OK) != 0 &&
        IsUnderPostmaster &&
        parse->commandType == CMD_SELECT &&
        !parse->hasModifyingCTE &&
        max_parallel_workers_per_gather > 0 &&
        !IsParallelWorker() &&
        !IsolationIsSerializable())
    {
        /* all the cheap tests pass, so scan the query tree */
        glob->maxParallelHazard = max_parallel_hazard(parse);
        glob->parallelModeOK = (glob->maxParallelHazard != PROPARALLEL_UNSAFE);
    }

afaict for deferrable READ ONLY DEFERRABLE transactions we could
trivially allow parallelism?  Am I missing something?

Greetings,

Andres Freund