Try to mitigate DNS failures when downloading stuff asynchronously

Try to mitigate DNS failures when downloading stuff asynchronously

* url-queue.el (url-queue-setup-runners): New function that uses
`run-with-idle-timer' for extra asynchronicity.
(url-queue-remove-jobs-from-host): New function.
(url-queue-callback-function): Remove jobs from the same host if
connection failed.