support updating multiple service workers coherently

[wanderview]

I believe at the f2f last week we decided that we need to provide some mechanism to update multiple service workers coherently. Right now if you have two service workers, one will always update before the other one.

I think a reasonable way to do this would be something like an "update group" in the registration.

navigator.serviceWorkers.register(script1, { group: 'foo' });
navigator.serviceWorkers.register(script2, { group: 'foo' });

Updates to either script would then trigger updates for all other scripts in 'foo' group. Each service worker in the group would progress through installing/waiting/activating together.