Valgrind : Leak Check / DRD : SubscriberTest possible memleak/data race

To reproduce locally:

```bash lint/valgrind.sh build/Release/bin/SubscriberTest```

```
==146203== LEAK SUMMARY:
==146203== definitely lost: 0 bytes in 0 blocks
==146203== indirectly lost: 0 bytes in 0 blocks
==146203== possibly lost: 411 bytes in 13 blocks
==146203== still reachable: 32,889 bytes in 420 blocks
==146203== suppressed: 0 bytes in 0 blocks
==146203==
==146203== For lists of detected and suppressed errors, rerun with: -s
==146203== ERROR SUMMARY: 4 errors from 4 contexts (suppressed: 0 from 0)
```

```
Tool: LEAK CHECK
------------------------
FAILURE DETECTED: ==156101== ERROR SUMMARY: 4 errors from 4 contexts (suppressed: 0 from 0)

Tool: DRD
------------------------
FAILURE DETECTED: ==156103== ERROR SUMMARY: 3 errors from 3 contexts (suppressed: 0 from 0)
```