Make auto update stats generic to Core_Upgrader

[nacin]

The statistics we've received from auto updates have been incredible for identifying major issues, error trends, confidence levels, etc. It triggered a number of excellent innovations in 3.7.

We should pass the same stats back for updates where "Update Now" is clicked as well. This will allow for comparisons between the two (as well as better identifying failure situations) and would help us make further improvements in 3.8. Being as informed as possible is critical to get to the point where we can be more confident in background updates for themes, plugins, languages, and major releases.

[dd32]

25657.diff​ is untested, and shifts the stats from the Background updater to the Core Updater, and sends along the type of update tried (upgrade, autoupdate, reinstall)

The change to update.php means that core version checks won't happen after plugin/theme installs/updates (aside from the one which we now fire in Core_Upgrader), I think we can probably skip that, as the built-in checks in wp_version_check() prevent it from making multiple useless requests.

[nacin]

25657.2.diff​ expands on the first patch. It changes how it sends the API data (much more sane now; the API has been updated to handle this) and also fixes a number of cases where a WP_Error for a rollback was not being treated as a special case.

[nacin]

In 25863:

Updates: Saner stats and rollback handling. WP.org now collects basic stats on non-auto updates as they have been hugely informative.

props dd32 for initial patch.
fixes #25657.