BridgeDB considers IPv6 clients in the same /64 to be "in the same subnet"

And an IPv6 /48 is rather trivial to obtain. When discussing this in the IRC channel, several people immediately spoke up to say that they have an IPv6 /48 subnet, which is equivalent to 65535 /64s. The current code (from legacy/trac#4297 (moved) and this commit) at bridgedb.Dist.uniformMap() would allow anyone with an /48 to pretend to be a maximum of 65535 clients to BridgeDB (which would still allow them to request IPv4 bridges, as well as Pluggable Transport bridges, I might add) and obtain a maximum of 65535 unique positions within a distributor's hashring per period, allowing the bridges within most hashrings to be entirely enumerated within a matter of a few hours.

As for fixing this bug, I planned to use (both for IPv4 and IPv6) whatever logic tor uses for the EnforceDistinctSubnets option. However, as it turns out, there may be a bug in that logic (legacy/trac#15518 (moved)) with respect to IPv6.

I propose (from talking to people, and glancing at https://en.wikipedia.org/wiki/IPv6_subnetting_reference and https://www.arin.net/resources/request/ipv6_initial_assign.html) that BridgeDB switch to treating IPv6 /32s (the minimum ARIN allocation for an LIR) as distinct subnets, and treat clients within the same /32 as coming from the same IP address.

[This was discovered while working on legacy/trac#4771 (moved) and legacy/trac#1839 (moved).]