This is another part in a series of patches that try to re-implement PR 14287 in smaller steps.
Besides converting `Driver._send` to use the Fetch API, this also changes the method to return a `Promise` to get rid of the callback function.
Please note that I *purposely* try to maintain the existing behaviour of re-sending the data on failure/unexpected response, including how/where the old callback function was invoked.
19c2cc8689