Experimental sync_assets feature to speed up asset compilation

This has been tested on a few large deploys and appears to work. I think it's safe to add it as an experimental option that we can try for a while.

* 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. 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 pre-building stuff like this on one server is underdeveloped. I'd like to figure that part out before unmarking this as experimental.