Returns next incoming message and removes it from the queue.
MC_OK is always returned.
Call this method to get the message from the input queue. This method returns only when the message is available (if the message is not available, the method doesn't return). Upon return the message is marked as being processed and the next call to GetMessage will return the next message.
Once the message was processed, the application must call MessageProcessed method to let the Messenger send reply (if necessary) and cleanup internal records.
This method together with PeekMessage and MessageProcessed is an alternative to using Queues.