Consider adding the IP address space to the network partition key

[letitz]

@annevk thoughtfully suggested this in #74. We could add the fetch client's IP address space to the network partition key used for fetches.

So far, it seems like the performance cost would be minimal. The vast majority of websites are public and would remain unaffected.

Such a change would present a small security benefit, as this approach would neatly prevent attacks using cache poisoning (see #74).

To me, the biggest benefit would be in simplifying the story around the HTTP cache (currently, we work around the lack of such partitioning by applying checks to the cache and invalidating entries in certain cases, which is complex), which would erase painful edge cases for users that frequently use the same website over public and private IPs. See for example https://crbug.com/1352591.