Add delete_misses/delete_hits to memcache

See https://github.com/memcached/memcached/blob/master/doc/protocol.txt,
there is delete_* stats we could use to report to datadog.

@remh , please help me out here. I am not sure if I am missing something, I cannot setup the environment to run the tests locally, and I couldn't find any page explaining how to do that. So I changed the code I thought would make sense.
The reason for this is, we need to display `delete_hits` and `delete_misses` to datadog, and right now we dont have those stats, and the memcached protocol says it support them.

Thoughts?