Upgrade for compatibility with EdgeX Foundry 2.x

[jrtitus]

Please refer to #8

• Upgrade to v2 packages
• Code organization for read handlers, write handlers, and configuration
• Ability to subscribe to multiple resources
• Ability to write configuration from consul to change subscriptions
• Ability to trigger OPC UA method calls
• Unit test coverage

  $ go test ./... -cover
  ?       github.com/edgexfoundry/device-opcua-go [no test files]
  ?       github.com/edgexfoundry/device-opcua-go/cmd     [no test files]
  ok      github.com/edgexfoundry/device-opcua-go/internal/config 0.011s  coverage: 100.0% of statements
  ok      github.com/edgexfoundry/device-opcua-go/internal/driver 2.724s  coverage: 71.3% of statements
  ?       github.com/edgexfoundry/device-opcua-go/internal/test   [no test files]

[jrtitus]

I just noticed that the resourceMap is for the resource level. If there are several Devices and Device Profiles created and running with this Device Service, does the resourceMap still work? Or, does this Device Service only support single OPC-UA Server (single Device and Device Profile)?

It only supports a single OPC-UA server, as it did in the 1.x version.

I have not tried it with more than one, but since the OPCUAServer section of configuration.toml specifies the DeviceName, I believe even if there were multiple devices listed, only the one specified would be used.

Monitoring multiple devices could probably be an enhancement, eventually. In that case the other existing service configuration options like Policy, Mode, CertFile and KeyFile would need to be part of the device configuration, I think, because each server could have different settings.