Use memcached_instance_st*  instead of memcached_server_instance_st for libmemcached-1.0.17

This is an alternative to PR #134 for fixing GH-133.

Instead of hacking in a typedef, it uses `memcached_instance_st*`, which is the documented type to use for `memcached_server_{name,port}`.

Fixes GH-133.