Prevent multiple simultaneous requests

[azaozz]

Prevents multiple simultaneous requests for the same plugin/theme if the user clicks on the link before the previous request is complete.

To test:

• Make sure the network is not extremely fast (like when testing on local install).
  Maybe add sleep( 5 ); to wp-config.php if needed.
• Open the browser tools, network tab, then click on the enable/disable link multiple times.

There should be only one request to admin-ajax.php at a time.

[audrasjb]

I tested this using a delay on my wp-config.php file and it worked fine.
Milestoning for 0.8.1.