plugin/forward: make Yield not block (#3336)

* plugin/forward: may Yield not block

Yield may block when we're super busy with creating (and looking) for
connection. Set a small timeout on Yield, to skip putting the connection
back in the queue.

Use persistentConn troughout the socket handling code to be more
consistent.

Signed-off-by: Miek Gieben <miek@miek.nl>

Dont do

Signed-off-by: Miek Gieben <miek@miek.nl>

* Set used in Yield

This gives one central place where we update used in the persistConns

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/forward/proxy_test.go

@@ -35,7 +35,7 @@ func TestProxyClose(t *testing.T) {
 		go func() { p.Connect(ctx, state, options{}) }()
 		go func() { p.Connect(ctx, state, options{forceTCP: true}) }()
 
-		p.close()
+		p.stop()
 	}
 }