Update the prefetch SW example to use cache-busting parameters

[jeffposnick]

R: @gauntface @wibblymat @PaulKinlan et al.

The current example showing off a service worker that prefetches URLs and then uses those responses in a cache-first strategy runs the risk of locking users in to a stale response indefinitely. To mitigate this risk, this change appends a cache-busting URL parameter to the fetch() request made in the install handler. That way, we can be assured that the response to that initial request is fresh at the time the install handler is run.

(It's possible that the resource might change after the install handler is run, and coordinating those resource changes with the service worker cache versioning is a separate problem that's already alluded to in the comments.)

[wibblymat]

👍