[relay] Refactor initial Relay connection (#2800)

Can support firewalls with restricted WS rules

allow to run engine without Relay servers
keep up to date Relay address changes

relay/client/picker_test.go

@@ -7,16 +7,19 @@ import (
 )
 
 func TestServerPicker_UnavailableServers(t *testing.T) {
+	connectionTimeout = 5 * time.Second
+
 	sp := ServerPicker{
 		TokenStore: nil,
 		PeerID:     "test",
 	}
+	sp.ServerURLs.Store([]string{"rel://dummy1", "rel://dummy2"})
 
 	ctx, cancel := context.WithTimeout(context.Background(), connectionTimeout+1)
 	defer cancel()
 
 	go func() {
-		_, err := sp.PickServer(ctx, []string{"rel://dummy1", "rel://dummy2"})
+		_, err := sp.PickServer(ctx)
 		if err == nil {
 			t.Error(err)
 		}