Disable client-side DNS cacheing by default

Proposal 205 discusses a proposal for making per-circuit DNS caches, but closes by suggesting that instead, client-side DNS cacheing simply be disabled.

This is necessary for avoiding some nasty linkability enabled by IPv6, and is probably a good idea for IPv4 users too.

Rather than just turn it off, I've written a branch to make it a set of per-client-port options, in case there's some really important reason it's needed.

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