It seems we'll end up using `block_on()` quite a bit, at least until
we're done transitioning to async, and the function name doesn't
conflict with anything else, so let's always import it when we need
it.

We need to let async-ness propagate up from the backend because
`block_on()` doesn't like to be called recursively. The conflict
materialization code is a good place to make async because it doesn't
depends on anything that isn't already async-ready.