Prop 306: Keep bridge IPv6 behaviour in sync with client behaviour

We should make some minor changes to Proposal 306, so that bridge IPv6 behaviour stays in sync with client IPv6 behaviour: https://gitweb.torproject.org/torspec.git/tree/proposals/306-ipv6-happy-eyeballs.txt

For an description of the necessary changes, see section 3.3.1 and 3.3.2 of the draft proposal 311: https://github.com/torproject/torspec/pull/103/files#diff-090a91ed4c8eac745c5ecc316b78dab4R153

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