monitor: add modification functionality

[jackchenjc]

closes #710

[magiconair]

This doesn't look right since something is appended when there was no match or am I missing something here? Also what happens when node.MonitoringParameters is nil? That also looks off.

Some minor style comments:

• You don't have to allocate an empty array in Go
• It's a common pattern to use early returns or continues as pre-conditions
• Prefer req over request since you're also using resp instead of response

How about this?

	var modifiedItems[]*ua.MonitoredItemModifyRequest
	for _, node := range nodes {
		for _, item := range s.itemLookup {
			if item.nodeID.String() != node.NodeID.String() {
				continue
			}

			// todo(fs): what happens if node.MonitoringParameters is nil? 
			// todo(fs): Should the request still be added?
			// todo(fs): If not, it could look like this
			//
			// if node.MonitoringParameters == nil {
			//     break
			// }
			// req := &ua.MonitoredItemModifyRequest{
			//	MonitoredItemID: item.id,
			//	RequestedParameters: node.MonitoringParameters,
			// }
			// req.RequestedParameters.ClientHandle = item.handle
			// modifiedItems = append(modifiedItems, req)
			// break
			

			req := &ua.MonitoredItemModifyRequest{
				MonitoredItemID: item.id,
			}
			if node.MonitoringParameters != nil {
				req.RequestedParameters = node.MonitoringParameters
				req.RequestedParameters.ClientHandle = item.handle
			}
			modifiedItems = append(modifiedItems, req)
			break
		}
	}

[jackchenjc]

You're right, using continue as pre-condition looks much better.
As for other styles, they are actually copied from the existing code (AddMonitorItems). Should I fix them as well in this PR?

I also agree that the request without node.MonitoringParameters should not be added. Thanks for the suggestion.

[magiconair]

As for other styles, they are actually copied from the existing code (AddMonitorItems). Should I fix them as well in this PR?

You're right. I didn't check the rest of the code. Consistency is more important. Keep them as you had it and just fix the pre-condition and the skip of the empty params. Thank you.

[magiconair]

@jackchenjc Thank you! I've merged it and released v0.6.5