Long queue time on the initial index.html request during SW update with heavy precache

Welcome! Please use this template for reporting bugs or requesting features. For questions about using Workbox, the best place to ask is Stack Overflow, tagged with `[workbox]`: https://stackoverflow.com/questions/ask?tags=workbox

**Library Affected**:
workbox-precaching

**Browser & Platform**:
all browsers

**Issue or Feature Request Description**:

notice long queue time on the initial index.html request when SW updates itself, especially when it requires a lot changes in precached assets, it seems the request on handling the index.html got blocked by all the other fetches happening on precache.

repro step is:

1. on a website that precache a lot files (hundreds of them)
2. clear the Cache but keep the service worker - you may also want to disable network cache
3. enable `update on reload` for SW
4. reload

our precache setting is very short:

```
precacheAndRoute(self.__WB_MANIFEST, { ignoreURLParametersMatching: [/.*/] })
```

tho the __WB_MANIFEST is quite a lot, like ~500 assets , and it does include the `index.html` itself, not sure if that's the issue that why `index.html` request itself got queued there - I would expect it will serve the old version with the old service worker before precache finished