memory leak for duplicated keys

[davies]

When a duplicated key is added into LRU, it will NOT call OnEvicted, which causes memory leak when OnEvicted is used to free memory or update counters.

[thrawn01]

I'm not sure if the maintainers are monitoring this repo anymore. We forked and fixed this in groupcache a few years ago. groupcache/groupcache-go@a139af4