subscription: add SetMonitoringMode functionality

[jackchenjc]

closes #711

Add SetMonitoringMode functionality and fix the potential issue that leaves the mutex locked.

Simply add the following code to examples/subscribe/subscribe.go can test this new feature

resp, err := sub.SetMonitoringMode(ctx, ua.MonitoringModeSampling, res.Results[0].MonitoredItemID)
if err != nil || resp.Results[0] != ua.StatusOK {
	log.Fatal(err)
}

[jackchenjc]

Not sure if we should update item.req.MonitoringMode while getting ua.StatusOK.

[kung-foo]

Ah, https://github.com/gopcua/opcua/blob/v0.5.3/README.md?plain=1#L206 needs an update as well. Otherwise LGTM