performance improves by around 100x or more
```
go test -v -run NONE -bench BenchmarkGetPartFile
goos: linux
goarch: amd64
pkg: github.com/minio/minio/cmd
BenchmarkGetPartFileWithTrie
BenchmarkGetPartFileWithTrie-4 1000000000 0.140 ns/op 0 B/op 0 allocs/op
PASS
ok github.com/minio/minio/cmd 1.737s
```
fixes#10520