[#376] netmap: Make attributes a non-pointer slice

The speed of copying (which is done regulary for e.g. subnet changes)
is less, however it isn't on the hot path and the absolute time
difference is insignificant.
```
name              old time/op    new time/op    delta
NodeAttributes-8    96.2ns ± 1%   158.3ns ± 1%  +64.61%  (p=0.000 n=10+10)

name              old alloc/op   new alloc/op   delta
NodeAttributes-8     32.0B ± 0%     32.0B ± 0%     ~     (all equal)

name              old allocs/op  new allocs/op  delta
NodeAttributes-8      2.00 ± 0%      2.00 ± 0%     ~     (all equal)
```

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

netmap/attributes.go

@@ -101,12 +101,10 @@ func WriteSubnetInfo(node *NodeInfo, info NodeSubnetInfo) {
 		}
 
 		if !presented {
-			var attr Attribute
-
-			attr.SetKey(key)
-			attr.SetValue(val)
-
-			attrs = append(attrs, &attr)
+			index := len(attrs)
+			attrs = append(attrs, Attribute{})
+			attrs[index].SetKey(key)
+			attrs[index].SetValue(val)
 		}
 	}
 
@@ -208,12 +206,10 @@ func IterateSubnets(node *NodeInfo, f func(refs.SubnetID) error) error {
 			}
 
 			// zero subnet should be clearly removed with False value
-			var attr Attribute
-
-			attr.SetKey(subnetAttributeKey(&id))
-			attr.SetValue(attrSubnetValExit)
-
-			attrs = append(attrs, &attr)
+			index := len(attrs)
+			attrs = append(attrs, Attribute{})
+			attrs[index].SetKey(subnetAttributeKey(&id))
+			attrs[index].SetValue(attrSubnetValExit)
 		} else {
 			entries++
 		}