v3api: Add a flag to RangeRequest to return only the keys.

Currently the user can't list only the keys in a prefix search. In
order to support such operations the filtering will be done on the
server side to reduce the encoding and network transfer costs.

etcdserver/etcdserverpb/rpc.proto

@@ -364,6 +364,9 @@ message RangeRequest {
   // a serializable range request is served locally without needing to reach consensus
   // with other nodes in the cluster.
   bool serializable = 7;
+
+  // keys_only when set returns only the keys and not the values.
+  bool keys_only = 8;
 }
 
 message RangeResponse {