code: @Nullable
      public static TtlRunnable get(@Nullable Runnable runnable) {
          return get(runnable, false, false);
      }
docs: Factory method, wrap input {@link Runnable} to {@link TtlRunnable}.
  
  @param runnable input {@link Runnable}. if input is {@code null}, return {@code null}.
  @return Wrapped {@link Runnable}
  @throws IllegalStateException when input is {@link TtlRunnable} already.