Happy Eyeballs: Use each fully resolved forwarding destination ASAP.

Happy Eyeballs: Use each fully resolved forwarding destination ASAP.

Do not wait for all request forwarding destinations to be resolved. Use
each resolved destination as soon as it is needed. This change does not
improve or affect IPv4/IPv6 handling (Squid still waits for both DNS A
and AAAA answers when resolving a single destination name), but the peer
selection code can now deliver each IP address to the FwdState/tunneling
code without waiting for all other destination names to be DNS-resolved.
TODO: Deliver A and AAAA answers to the peer selection code ASAP.

This change speeds up forwarding in peering environments where peers may
need frequent DNS resolutions but that was not the motivation here.

This change is a step towards a more complete Happy Eyeballs support in
Squid. The general path can be roughly summarized as follows:

1. Squid has already supported: Use parallel A and AAAA queries.
2. This change: ASAP delivery of IPs from peer selection to FwdState.
3. The next step: ASAP delivery of IPs from DNS to peer selection.
4. A separate project may add: Use parallel TCP connections.

Also fixed missing cbdataReferenceDone(psstate->callback_data) calls
in three error handling cases. These cases probably leaked memory.