jumbo patch from 39380 - and moving things 'up' to mod_cache itself

[Dirk-Willem van Gulik]

Niklas,

There is a lot of valuable stuff in your jumbo patch - but I am not  
sure what the best approach is to fold it in.

Could you have have a look at the rough patch I posted earlier today -  
and let me know if you have any thoughts
as to which parts should be moved 'up' -- and hence be of use to other  
caching backends as well - and which
parts are pure disk optimized/specific ?

Or perhaps form an option if we need multiple disk cache modules -  
each optimized differently (e.g. for large files on multiple spindles;  
versus very 'hot' cache which is virtually living on a meory disk).

Thanks,

Dw