Some read requests do not honor the store labels

## Bug Report

TiDB only set `MatchStoreLabels` on `executorBuilder.getSnapshot`, however some of executors may use `txn.{Get,BatchGet}` directly. These reads do not honor the store labels thus may access replicas on the slow tikv.

### 1. Minimal reproduce step (Required)

Deploy a cluster with 3 AZs and evict-slow-store-scheduler, run a stale read workload, and then slowdown a tikv in one AZ.

### 2. What did you expect to see? (Required)

The other two AZs should not be affected after leaders have been evicted.

### 3. What did you see instead (Required)

All AZs had perf regression during the fault injection.

### 4. What is your TiDB version? (Required)

release-6.5