Delay the cache write in StrategyHandler's fetchAndCachePut()

[jeffposnick]

In the course of debugging some performance related issues with a Workbox-based service worker that uses a CacheFirst strategy for a burst (1000+) of requests, @wanderview suggested that we should add in a short delay before we write to cache in StrategyHandler's fetchAndCachePut():

https://github.com/GoogleChrome/workbox/blob/1deeba932904bcea52ea9fe809e0aa5cc74b0941/packages/workbox-strategies/src/StrategyHandler.ts#L253-L260

I don't think we should do this outside of a major semver change, but it might be something to play around with as part of a Workbox v7 pre-release and get a feel for whether there are any unintended consequences.

[wanderview]

Or possibly an option with some documentation explaining why you might want to add delay.