Fetch pages 2..10 in parallel

Closes #1944

Closes #1786

We were only caching GET requests that start with `repos/` and so this skipped URLs requested by our release-notes (CHANGELOG.md) function.

Set env variable RENOVATE_PAGINATE_ALL to disable Renovate’s regular limiting of 1000 results (unlimited results will be returned).

This reverts commit dd26a018.

Refactors all instances of .concat() to use array spread operators instead

Closes #1459

Rename writeToken to forkToken to make it clearer. This enables GitHub API to use one token for all reads, and another token just for writing to the forked repository.

The existing github wrapper is now extended to cache any get request. The existing pr cache is removed as it is now redundant.

Refactors file system of platforms and adds test to ensure GitHub and GitLab have same module.exports signatures.

Closes #996

Now will work with any GitHub host, instead of just api.github.com

From https://developer.github.com/v3/apps/available-endpoints/:
> In order to access the API with your GitHub App, you must provide a custom media type in the `Accept` Header for your requests.
```
application/vnd.github.machine-man-preview+json
```

* fix: add upgrade and branch timings to log

Adds timing logs for upgrade detection + branch updating stages.

Closes #821

* improve request retry logging