Add a status report check for available memory and evictions

For many of the sites we work on, we end up sizing memcache so that evictions should never occur. In fact, most of the time we see evictions it's because of a misconfiguration with memcache set to only allocate the default 32 or 64MB of memory.

While it's not always an issue, it would be nice if the memcache module could raise requirements warnings if it looks like the memory should be increased.

A suggested implementation plan:

1. Refactor all of the "API" private methods, like statsMemory() out from MemcacheStatisticsController into a new Statistics class in the memcache module itself.
2. Use that statistics class with various get methods instead of the raw $stats array that's currently there. Since it's all private methods (yay!) we can do this without a b/c break.
3. Finally, use the new statistics class in memcache_requirements() to throw the right warnings.