…gUpdates: APIs.

Over time, there have actually been a lot of legitimate uses for an API like this.
In fact, I'm not quite sure what has held us back from adding one!

I believe that at least some portion of -wait calls (even if less than 50%) could be
replaced with -onDidFinishProcessingUpdates:, which could potentially improve the
performance of applications using -wait by a significant amount.

Please take a close look at implementation correctness. Although I'm in a bit of a
rush, I'm aiming to make this properly documented and added a basic test -- but it
could certainly use some more detailed testing as a followup.

…gUpdates and -onDidFinishProcessingUpdates:

…ngUpdates:. Rename -waitUntil to consistent naming.