Implemented stats and flush_all

To implement stats and flush_all, I had to add AllServers() to ServerSelector. FlushAll() sends flush_all to all servers, Stats(index int) allows to select a server. Not the most elegant way, especially since ServerSelector.AllServers() returns the server in an implementation specific way. But ServerList is maintaining the server order, and for single server settings it's always Stats(0).

I also added memcache_stats as simple demo and utility.