Change subscribe_messages to yield individual messages by default

[agrare]

Currently subscribe_messages yields the entire batch of messages, this can be useful if you intend to process the entire block of messages but it isn't helpful if you want to process one message at a time.

Specifically acking messages can only be done after the entire block of messages has been processes, rather than after each message.

[bzwei]

We can add a new method for single message subscription while leaving the existing batch subscription untouched.
Not all message clients support single message subscription. My understanding QPID for ActiveMQ does not. We don't have AMQP implementation yet.