Introduce abstract queue terminus within the indirection system.

The queue abstract terminus allows the standard indirector behaviors to interact with a message queue broker, such that the indirector's "save" method writes the relevant model object out to a queue on the message broker. While the indirector's "find" method does not map to a message queue, the queue terminus class offers a "subscribe" method that allows for easy implementation of an event loop, receiving indirected objects saved to a queue as they come in.