Happy Eyeballs: Do not discard viable reforwarding destinations (#567)

Happy Eyeballs: Do not discard viable reforwarding destinations (#567)

When HappyConnOpener starts opening two connections, both destinations
are removed from the shared destinations list. As soon as one connection
(X) succeeded, the other destination (Y) was essentially forgotten. If
FwdState, after using X, decided to reforward the request, then the
request was never reforwarded to Y. We now return Y to the list.

Also abort the still-active ConnOpener job upon the termination of its
"parent" HappyConnOpener job. Without an explicit termination, those
abandoned child jobs wasted OS and Squid resources while the OS was
trying to open the requested connection. They would terminate on a
timeout or when the connection was finally opened (and discarded).

Waiting for the child ConnOpener job to end is a useful optimization,
but it remains a TODO. It requires complex accumulation avoidance logic.

Also fixed retrying via FwdState::fail() which could reorder addresses.
It incorrectly assumed that only the prime connections were retried.