Move Jetpack endpoints to individual files.
Improve update_core function.

By default try to find the latest core version.
Restore single upgrade version.
Oops copy and paste.
Fix update plugin test.
Add Update Core Test

Leaving it commented as it could change the state of the site.
Merge pull request #1144 from Automattic/jetpack-endpoints-refactoring

Jetpack endpoints refactoring. Move Jetpack endpoints to individual files.