Accept a command from the adapter's service and perform some
internal action based upon it. CommandMessages are used for messages
which control the state of the connection between the client and
the server. For example, this lets the adapter perform processing with
subscribe, unsubscribe, and poll operations. For subscribe and unsubscribe,
this method is only called if handlesSubscriptions returns true.
The service will perform some processing on the message before and after it
calls this method. For subscribe messages the MessageService
will register a subscription after this method returns successfully.
For unsubscribe messages, the MessageService will unsubscribe after this
method returns successfully. For both of these messages, this method
can return null or it can return the AcknowledgeMessage to send to the client
for the reply to this operation. If a MultiTopicConsumer is used on the
client, this method will receive a MULTI_SUBSCRIBE message.
For POLL operations, this method can return a list of messages to be
added to the set returned to the client for this poll. If it returns
null, it means no messages are to be added to the set already queued
up for this client.