MapiResponse never gets garbage collected in Node

Nobody has claimed this yet.

We have a memory leak issue with a Node app that appears to be caused by MapiResponse never getting cleaned up by the garbage collecter. We are creating a client and using to get directions as below using Node v12.16:
```
const mapboxDirectionsFactory = require('@mapbox/mapbox-sdk/services/directions');

class MapboxAdapter {
constructor(options) {
this.client = mapboxDirectionsFactory(options);
}
directionsConsideringTraffic({geoLocations}) {
return this.client.getDirections({
profile: 'driving-traffic',
geometries: 'geojson',
steps: true,
waypoints: geoLocations.map(coordinates => ({coordinates}))
}).send();
}
}

module.exports = {
MapboxAdapter
};
```

But we can see that `MapiRequest` and `MapiResponse` never get cleaned up. Looking at heap snapshots from prod we can see these just keep growing (example here has grown up to 90MB just for `MapiResponse`)

I have also reproduced running locally in the Chrome debugger. I can see these objects build up and never get removed even when I manually run a GC in Chrome.

It looks like the SDK uses the global namespace so I guess this is why they are never GC'd? Is there something special we should be doing in Node to avoid this issue?

Start by reproducing the directions call in the issue's MapboxAdapter example with Node v12.16, then inspect heap snapshots and forced-GC behavior for retained MapiRequest and MapiResponse objects. Done means identifying the retention path and confirming that repeated requests no longer cause these objects to grow indefinitely.

Written by the indexing model from the issue text.