Improved handling of URLs to be purged for better performance.

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.

This idea was taken from a derivative work (apparently) of
wp-varnish: https://github.com/admingeekz/varnish-wordpress

The code could be further enhanced to send PURGE/BAN requests
using HTTP pipelining to further improve performance.
Added changelog entries.
Complete rewrite of the part that generates the URL to be purged.
Merge branch 'enhance-common-objects' into gnotaras-base