Selection between multiple external IPs should consider reachability

[FelixMcFelix]

Currently, an instance having several floating IPs will choose one using only the hash of the flow-key (VPC-local IP, dst IP, proto, src port, dst port). This assumes that all IPs assigned to an instance are treated identically upstream for the purposes of routing and reachability of destinations.

Part of this is that different IP pools might encode different semantics to an end-user: Internet connectivity, internal-only IPs, or for exclusive VPN tunneling to a cloud host. We should be making use of routing table information to determine which IPs should be selected as a source for a given destination address.