[patch] cache evictions stats

[marc at corky.net]

A very important stat.   +1 for inclusion.

> Here is a patch (against 1.2.1) to count the number of non expired 
> keys that are evicted from the LRU.
>  
> This is the only way I know to check if memcached has enough memory, 
> and tracking this is something I always have missed in memcached.
>  
> My C is a little bit rusty, and this is my first memcached patch, so 
> please be indulgent.
>  
> Jean-François Bustarret
> WAT - CTO
> http://www.wat.tv
>