Ensure multiple consumer follows same processing logic as normal consumer

[McSimp]

Currently MultipleConsumer does not handle exceptions in the same way as the regular Consumer since it uses its own processQueueMessage instead of processMessage, which does not include handlers for exceptions like StopConsumerException.

I think that the MultipleConsumer should probably have the same behaviour for these exceptions, so I've placed the relevant code in processMessageQueueCallback (I'm open to a better name for this) which is called by processMessage in Consumer and processQueueMessage in MultipleConsumer. This way when new functionality is added, both consumers should continue to have the same behaviour.