obfs4: stop obfs4 client and proxy in a go routine

the Stop is blocking and cannot be called concurrently
with Start, when for some reason obfs4 client fails to
connect and has not yet returned from Start, a call to
Stop will block until Start has returned, which  makes
UI unresponsive for some time

calling it in a separate Go routine works around  this
issue as we can proceed to stop the openvpn process

pkg/vpn/openvpn.go

@@ -376,11 +376,18 @@ func (b *Bitmask) StopVPN() error {
 	if err != nil {
 		return err
 	}
+	// try to stop obfsvpn client in a go routine
+	// as the stop operation takes time and might
+	// block if start didn't return yet
 	if b.obfsvpnProxy != nil {
-		if _, err := b.obfsvpnProxy.Stop(); err != nil {
-			log.Debug().Err(err).Msg("Error while stop obfsvpn proxy")
-		}
-		b.obfsvpnProxy = nil
+		go func() {
+			if _, err := b.obfsvpnProxy.Stop(); err != nil {
+				log.Debug().
+					Err(err).
+					Msg("Error while stop obfsvpn proxy")
+			}
+			b.obfsvpnProxy = nil
+		}()
 	}
 	b.tryStopFromManagement()
 	if err := b.launch.OpenvpnStop(); err != nil {