Fix MongoClient deprecation warning
Fix MongoClient deprecation warning

This edit fixes this warning
```
(node:8234) DeprecationWarning: current Server Discovery and Monitoring engine is deprecated, and will be removed in a future version. To use the new Server Discover and Monitoring engine, pass option { useUnifiedTopology: true } to the MongoClient constructor.
```
mcollina/aedes-persistence-mongodb