cache cleaning logic

[Nick Shaforostoff]

>>also, i would like to add gzip compression of cache entries (for
>> example, i got 3 times size decrease on my cache)
>Makes sense. If you can, make the compressing an optional, background, 
>low-priority task.

I've already implemented gzipping khtml internal history cache (btw, max 12 pages in size),
and it adds 0-2 _m_sec on http://doc.trolltech.com/4.3/qstring.html
CPU load doesnt even reach 50% (i have 1.8ghz).

If the file is big, it will be even faster, because less data has to be read from (slow) harddrive.

and, of course, i doubt that old (slow on gunzipping) systems will have enough memory to run KDE! :)))