Valgrind : DRD / Helgrind : RpcClientTest possible data race

To reproduce locally:

```
bash lint/valgrind.sh build/Release/bin/RpcClientTest
```

```
Tool: DRD
FAILURE DETECTED: ==155836== ERROR SUMMARY: 93332 errors from 453 contexts (suppressed: 52 from 28)

Tool: HELGRIND
------------------------
FAILURE DETECTED: ==155889== ERROR SUMMARY: 55128 errors from 338 contexts (suppressed: 26123 from 83)
```