Given we're no longer using dependent strings in
LookupCacheV4::PrefixString(), we will end up make a copy of the
prefixes at some point. Let's do it early and remove a bunch of
complicated code.
Make the string copies fallible so that we return an error and
fail the update instead of crashing.
MozReview-Commit-ID: 5cZHSDIJSlD
170af9408f