memdb snapshot getter and iter returns latest data instead

[you06]

The snapshot getter and iter of memdb should read bypass the staging data. But the current snapshot read the latest data instead, see the unit test in PR.

TiDB unionscan executors read data from snapshot:

• https://github.com/pingcap/tidb/blob/cdd7c9e3044e2f4cc1dce611d0786a4f467bbeb4/pkg/executor/union_scan.go#L113
• https://github.com/pingcap/tidb/blob/7e73ddc91b5f9f089e84f0dc645cf71a27b2ad50/pkg/executor/mem_reader.go#L578-L582