So far, BridgeDB's distribution of vanilla IPv6 bridges was broken.
When a user would request one, BridgeDB would use the following two
filters to select bridges:
* byTransportNotBlockedIn(None,us,6)
* byProbingResistance(vanilla,6)

Neither filter (correctly) filtered for IPv6.  It is not enough to check
bridge.address because it's the bridge's IPv4 address.  We need to check
bridge.allVanillaAddresses because a bridge's IPv6 address is advertised
in its "a" consensus line.

Note that this patch only fixes IPv6 distribution for *vanilla* bridges.
Pluggable transports bridges are currently unable to have dual-stack
support for both IPv4 and IPv6.  See the following ticket for more
details: <https://bugs.torproject.org/11211>

This fixes <https://bugs.torproject.org/26542>.