A storage service prototype based on MongoDB

This is an initial prototype for a storage back-end using MongoDB.  This would provide a way to fill in those key-based methods that are currently unimplemented, and provide a more straightforward view into the data.  The schema is pretty simple.  Every record in the database will look like this:

```
{
  'simulationId' : 'simulation-uuid',
  'instanceId' : 'instance-uuid',
  'iteration' : 0,
  'keys' : 
  {
    'key1' : 'value1',
    'key2' : 'value2',
    ...
  }
}
```

It's still a bit rough around the edges.  It's mostly an exploratory prototype into how MongoDB works.

And I wanted to see how these pull requests work.
We will need to eliminate the dependency on an external server before I will be able to pull this code. I see an embedded MongoDB at https://github.com/flapdoodle-oss/de.flapdoodle.embed.mongo it might be good enough to use for testing purposes.