get unexpected key using scan api

Describe the bug
Invoke the scan api of the txn client with startKey, obtain unexpected data that out of range

for example, there are two key k|d|0 and k4|d|0 , invoke scan with startKey k4|d| ,
but get unexpected key k|d|0 additionally.


Java Client and TiDB/TiKV version info
client version: 3.1.2
tikv version: v4.0.15!

Keys are lexically ordered, therefore k|d|0 is greater than k4|d.

4: 0x34
|: 0x7c