* Make sure reading the body of same JSON page request safe.
It's not possible to read the body twice from fetch()'s response
So, we've to clone the response before doing anything.
* Fix tests.
* Implement a very simple prefetching solution.
* Remove next-prefetcher.
* Require 'whatwg-fetch' only in the client.
* Use xhr in the code.
* Use a simple fetching solution.
* Fix 404 and xhr status issue.
* Move the prefetching implementation to next/router.
* Add deprecated warnning for next/prefetch
* Run only 2 parellel prefetching request at a time.
* Change xhr to jsonPageRes.
* Improve the prefetching logic.
* Add unit tests covering the Router.prefetch()
* Update examples to use the new syntax.
* Update docs.
* Use execOnce() to manage warn printing.
* Remove prefetcher building from the flyfile.js
Because, we no longer use it.
