Improved handling of URLs to be purged for better performance.

This patch does not change anything else than the handling of the URLs to be purged.

Instead of placing a PURGE/BAN for each URL as it is generated, all URLs are collected in an array. After removing duplicate URLs from the array, a PURGE/BAN is then placed for each URL. Using this method, we avoid sending duplicate PURGE/BAN requests to the varnish servers and improve performance.

Also, I had to fix the indentation of the WPVarnishPurgePool() function to make it easier for me to set up the loop correctly avoiding unnecessary duplicate collection of the varnish server information or other URL calculations.

Finally, I added changelog entries under the ``dev`` release. Please change this accordingly.

Thanks in advance.