identity: Use clear to clear the finder seen map

Faster:

```
Finder/Find_one-10    172.8n ± 26%   129.3n ± 2%  -25.18% (p=0.002 n=6)
Finder/Find_none-10   174.0n ±  1%   130.6n ± 0%  -24.94% (p=0.002 n=6)
```

identity/finder.go

@@ -45,9 +45,7 @@ func putSearchID(sid *searchID) {
 	sid.dp = nil
 	sid.peq = nil
 	sid.eqer = nil
-	for k := range sid.seen {
-		delete(sid.seen, k)
-	}
+	clear(sid.seen)
 	searchIDPool.Put(sid)
 }