SubConn transitions directly from CONNECTING to IDLE

[zasweq]

There is a race when a Subchannel goes from CONNECTING to READY to IDLE that prevents the READY state change notification. We should fix this, as this is causing specific case handling in balancers such as the new pick first leaf policy for DualStack.

[arjan-bal]

The original issue that I opened: #7503

The problem is that the health checking go routine may report a failure before the ready update is sent:

grpc-go/clientconn.go

Lines 1406 to 1412 in b45fc41

	if hctx.Err() != nil {
	// onClose was already called for this connection, but the connection
	// was successfully established first. Consider it a success and set
	// the new state to Idle.
	ac.updateConnectivityState(connectivity.Idle, nil)
	return nil
	}

Once we remove health checking from addrConn, the edge case should no longer happen.