Non-linear unmarshal performance

[DanG100]

go test ./ygnmi/ -bench ^BenchmarkLookupListParent$ -benchtime=2000x

goos: linux
goarch: amd64
pkg: github.com/openconfig/ygnmi/ygnmi
cpu: Intel(R) Xeon(R) CPU @ 2.80GHz
BenchmarkLookupListParent-48                2000           1750459 ns/op

goos: linux
goarch: amd64
pkg: github.com/openconfig/ygnmi/ygnmi
cpu: Intel(R) Xeon(R) CPU @ 2.80GHz
BenchmarkLookupListParent-48                4000           3387316 ns/op