Add IPv4/IPv6 connections to the heartbeat

When we implement IPv6 extends, we'll need to monitor how many IPv4 and IPv6 connections a relay is making, using every combination of:

• IPv4/IPv6
• initiate/accept

We may also want to add the reason that the connection was initiated:

• IPv4/IPv6
• reachability (origin, authorities only) / client (origin, authorities and relay also act as clients) / extend (relays and authorities only)

It's probably safe to also count all reachability circuits, but it might not be safe to count client circuits on clients, or extend circuits. Let's think about the privacy implications before adding these to the heartbeat.