Fetch service worker scripts with "no-cache" by default (#1020)

This introduces service worker registration's use cache field and its
related APIs: options.useCache to register method and
registration.useCache for ServiceWorkerRegistration objects. This
changes the default cache mode of fetching SW scripts to "no-cache".
After the change, 24 hours limit and job's force bypass cache flag rules
are still enforced. register() method's options value set to
{ useCache: true } re-enables the previous default behavior provided
before this change.

Fixes #893 #894.