Now adds `stdout` and `stderr` from `npm` and `yarn` installs to the "Generated lockfile" log message, so that the exact versions of npm and yarn can be known in the default logs.

Closes #827

* fix: add upgrade and branch timings to log

Adds timing logs for upgrade detection + branch updating stages.

Closes #821

* improve request retry logging

`npm` and `yarn` lockfile generation use promisified child process `exec` now instead of `spawnSync`.

This fix improves the way Renovate detects embedded/installed npm and yarn. It tries:
- locally installed npm or yarn
- npm or yarn embedded inside globally installed renovate
- globally installed npm or yarn
- global `yarn` or `npm` commands as fallback

Fixes #824 

* chore(deps): update dependency prettier to v1.6.1

* chore(deps): update dependency prettier to v1.7.0

* run lint

* chore(deps): update dependency eslint-config-airbnb-base to v12.0.0

* fix lint

* install npm from npm

* use embedded npm and remove versions checking

Renovate will now depend on yarn like any other dependency and call this version directly, instead of having an embedded version. This will allow us to "renovate" the yarn version we use too.

- Any package names not matching valid pattern will be rewritten to “dummy”
- “engines” and “scripts” fields will be completely removed

Closes #801, Closes #784