Re-implement update_sql as query logger

So if someone wants to tackle this, here's approximately how I'd go about it:

* In the update process, before each update hook runs, start a new logger.
* After the update hook, stop the logger and grab its saved data. Store that to some data structure.
* Take the return from the update hook as an array of *extra* messages. Allow nothing to be returned, in which case there's no extra messages.
* When all hooks are done running, iterate through the data structure and make a list of each update hook and the queries it ran and any messages it returned.

Submit that patch first so we can get it committed. Then we can exterminate update_sql and modify the schema API to not bother with the stupid $ret array that gets passed around everywhere in separate patches.

So who wants to be a hero? :-)