Valgrind : Leak Check / DRD : UTransportMockTest possible mem leak, and data race

To reproduce locally:
```
bash lint/valgrind.sh build/Release/bin/UTransportMockTest
```

```
Tool: LEAK CHECK
------------------------
FAILURE DETECTED: ==156385== ERROR SUMMARY: 4 errors from 4 contexts (suppressed: 0 from 0)

Tool: DRD
------------------------
FAILURE DETECTED: ==156681== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
```

```
==146334== LEAK SUMMARY:
==146334== definitely lost: 0 bytes in 0 blocks
==146334== indirectly lost: 0 bytes in 0 blocks
==146334== possibly lost: 868 bytes in 27 blocks
==146334== still reachable: 43,006 bytes in 548 blocks
==146334== suppressed: 0 bytes in 0 blocks
==146334==
==146334== For lists of detected and suppressed errors, rerun with: -s
==146334== ERROR SUMMARY: 4 errors from 4 contexts (suppressed: 0 from 0)
```