may you add default replicas ?

1.add default replicas
groupcache/consistenthash.go: line 34.
replicas must has a default value. if replicas <=0, then it will do nothing when call `func (m *Map) Add(keys ...string)`

and i suggest the default replicas value is 23

2.add sync.Mutex. when call Add and Get，we can lock the kes and hashMap
