Allow request cancelation

[sarahsnow1]

We can update HttpHandler#requestRoute to return a Call object (or wrapper around it) to support request cancellation:
https://square.github.io/okhttp/3.x/okhttp/

[ecgreb]

I would rather expose our own public API for request cancellation rather than directly using Call. Upgrading OkHttp required a number of changes in the Mapzen SDK. Ideally upgrading this library's inner dependencies would not require changes to the public API nor any apps using it.

[sarahlensing]

I prefer that too