Rough draft of sync_assets feature (speeds up asset compilation)

This feature is hacked in as a logic branch right now. I don't think it's the right way to implement it but it *has* been tested on a few large sites and appears to work smoothly.

My reason for hacking it like this rather than adding, for example, a strategy that does it:

* This doesn't change the choice of private vs shared vs shifting assets. Assets can still be managed in the way an application prefers with or without this features, so picking a strategy to use for asset sync shoehorns everyone. (This may be what I end up doing in the long term, since there's likely a "winner" of the strategies. Private is my current favorite for isolation and rollback tolerance.)
* The mechanism I have available for running pre-build stuff like this on one system is underdeveloped. I'd like to figure that part out before committing to an implementation.