Allow to prioritize some view updates over others

An important feature for systems with large numbers of items and high throughput is to be able to prioritize different views (by
design doc, for example). A clear use case is when promoting development view to production, you need to ensure that the view indexing that happens in the background for the updated production view has lower priority than the indexing that is handling live traffic for current clients.

That is, promoting a development view to production (which will be churning through all of the docs and causing a lot of processing) should not starve out view updates that are handling live traffic. It should be low enough priority that clients querying views don't notice any appreciable slowdown due to the large production view update.