Fixed getServerByKey function

The underlying function memcached_server_by_key returns a memcached_server_instance_st instead of memcached_server_st. The structures were compatible to each other, but this
recently changed between 1.0.5 and 1.0.13. This made this function failing.

See https://bugs.launchpad.net/libmemcached/+bug/1076048 for more info.

By using the proper function calls they work again, but unfortunately there's no fix for the weight. I think it's useless to return the weight, since you can know it from php-user-land (it's passed to either addServer or addServers). I suggest to remove the weight as a return type, but I didn't want to break anyone's code.