Segmentation faults when stress testing
I've been stress testing OpenMonero to try and understand the max limits of (1) concurrent imports and (2) search threads. When testing either scenario, I've sporadically run into OM crashing. Here's a snippet grabbed while running 50 imports:

```
2019-07-26 14:49:02,753    I Current blockchain height: 1887004, pool size: 0 txs, no of TxSearch threads: 50
2019-07-26 14:49:13,727    I Current blockchain height: 1887004, pool size: 1 txs, no of TxSearch threads: 50
Segmentation fault (core dumped)
ubuntu@ip:~$ echo $?
139
```

No other errors seem to be produced and none of the system resources seem to be maxed out. Interestingly, I'm not always able to replicate under the same conditions -- spinning up 5000 search threads for (2) will sometimes run successfully for hours, while other times OM will crash after 20 minutes. 

I'm running from commit `1e6c13f` on the `devel` branch, synced to mainnet. Please let me know what I can do to help debug!