Move Ember Data FastBoot support to Ember Data itself

We are adding direct support for `FastBoot` in Ember Data.

https://github.com/emberjs/data/pull/5385

We will have two paths: `jQuery.ajax` which will require `najax` and a new, opt-in `ember-fetch` backend which have built in `FastBoot` support and do not require `najax`.

I don't think we can/should drop MonkeyPatch™ from `ember-cli-fastboot` right away because of support for older `ED`. But I am interested to hear what would be an acceptable time frame for doing it (@rwjblue)?