SetProto for protoSink write-through

Hi,

Are there any plans to implement more efficient (write-through or copy) SetProto for protoSink, as described in the comments here: https://github.com/golang/groupcache/blob/master/sinks.go#L198-L201 ?

The current implementation is similar to using a byte slice sink and marshalling the object before inserting it into the cache and unmarshalling when it is returned from the cache.