[#324] Close nns resolver after use

Signed-off-by: Marina Biryukova <m.biryukova@yadro.com>

internal/frostfs/util/util.go

@@ -28,6 +28,7 @@ func ResolveContractHash(contractHash, rpcAddress string) (util.Uint160, error)
 	if err := nns.Dial(rpcAddress); err != nil {
 		return util.Uint160{}, fmt.Errorf("dial nns %s: %w", rpcAddress, err)
 	}
+	defer nns.Close()
 
 	return nns.ResolveContractHash(domain)
 }