image-dired: Increase number of asynch thumbnail generation jobs

image-dired: Increase number of asynch thumbnail generation jobs

* lisp/image-dired.el (image-dired-queue-active-limit): Increase the
number of asynchronous thumbnail generation jobs to a maximum of 4 or
processor count divided by 2.  This makes thumbnail generation around
30 % faster when benchmarking this on my machine (8 cores/4 jobs).

(image-dired-debug): New defvar.
(image-dired-debug-message): New defun.

(image-dired--generate-thumbs-start): New defvar.
(image-dired-create-thumb-1, image-dired-display-thumbs): Display time
spent on thumbnail generation if 'image-dired-debug' is non-nil.