feat update default fiber stack=4 m

  // Config holds scheduler configuration settings that can be passed to the

  // Scheduler constructor.

  struct Config {

    static constexpr size_t DefaultFiberStackSize = 1024 * 1024;

    static constexpr size_t DefaultFiberStackSize = 4 * 1024 * 1024;

    // Per-worker-thread settings.

    struct WorkerThread {

    // spinForWork().

    void waitForWork() REQUIRES(work.mutex);

    // spinForWorkAndLock() attempts to steal work from another Worker, and keeps

    // the thread awake for a short duration. This reduces overheads of

    // spinForWorkAndLock() attempts to steal work from another Worker, and

    // keeps the thread awake for a short duration. This reduces overheads of

    // frequently putting the thread to sleep and re-waking. It locks the mutex

    // before returning so that a stolen task cannot be re-stolen by other workers.

    // before returning so that a stolen task cannot be re-stolen by other

    // workers.

    void spinForWorkAndLock() ACQUIRE(work.mutex);

    // enqueueFiberTimeouts() enqueues all the fibers that have finished