Add key-hit and key-miss to keyspace notification.

Hi,

I have committed small changes for adding key-hit and key-miss notification.

Redis has keyspace notification feature, but doesn't contain key hit (read) miss event. I think, this change can provide to track that on server side.

* k : Key hit event. (events generated when a data of the key is read)
* m : Key miss events (events generated when a key is not exists in db)

I didn't put above 2 event into 'A' alias, due to avoid side effects to current users. These probably flow pour many events enough. Here is what I added comment in redis.conf

#  A     Alias for g$lshzxe, so that the "AKE" string means all the events.
#        (Key hit events and key miss events are not included this Alias.
#         That must add particularly in the config string like "AKEkm".)

Thanks.
Jinoos