plugin/forward: move Dial goroutine out (#1738)

Rework the TestProxyClose - close the proxy in the *same* goroutine
as where we started it. Close channels as long as we don't get dataraces
(this may need another fix).

Move the Dial goroutine out of the connManager - this simplifies things
*and* makes another goroutine go away and removes the need for connErr
channels - can now just be dns.Conn.

Also:

Revert "plugin/forward: gracefull stop (#1701)"
This reverts commit 135377bf77.

Revert "rework TestProxyClose (#1735)"
This reverts commit 9e8893a0b5.

plugin/forward/forward.go

@@ -119,7 +119,7 @@ func (f *Forward) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg
 
 		if err != nil {
 			// Kick off health check to see if *our* upstream is broken.
-			if f.maxfails != 0 && err != errStopped {
+			if f.maxfails != 0 {
 				proxy.Healthcheck()
 			}
 
@@ -185,7 +185,6 @@ var (
 	errNoHealthy     = errors.New("no healthy proxies")
 	errNoForward     = errors.New("no forwarder defined")
 	errCachedClosed  = errors.New("cached connection was closed by peer")
-	errStopped       = errors.New("proxy has been stopped")
 )
 
 // policy tells forward what policy for selecting upstream it uses.