MQTT sending concurrency error

[weichou1229]

In high-frequency usage, the MQTT sending function might crash due to concurrent operation.

For example, the app-service receive two events and try to export data via MQTT, the first event and the second event initialize the MQTTClient concurrently and cause some network error.

Code snippets:
MQTTSender https://github.com/edgexfoundry/app-functions-sdk-go/blob/master/pkg/transforms/mqtt.go#L122
MQTTSecretSender https://github.com/edgexfoundry/app-functions-sdk-go/blob/master/pkg/transforms/mqttsecret.go#L240

if sender.client == nil || sender.secretsLastRetrieved.Before(edgexcontext.SecretProvider.SecretsLastUpdated()) {
	err := sender.initializeMQTTClient(edgexcontext)
	if err != nil {
		return false, err
	}
}