feature: abstract the async queue

[peterschutt]

Thinking of having a visitor method on the service object, so when an operation is performed on the resource, the object and operation type are put on the async queue. The worker instantiates the service object instance and passes the object and operation to the visit method. This way we can keep all the business logic in the service object and keep the configuration of the queue and worker minimal.

As this stuff is all intra-application, thinking about using pickle for serializing the model instance so that we don't need to serialize to json and deserialize back to sqlalchemy model. Should really do some profiling on this before doing it though as if not a big win then prob not worth the WTF factor.

[peterschutt]

To Do:

• profile pickle vs json for serialization.
• build the worker function, it will need to import the service object, instantiate it, and call the callback method.
• build the enqueue mechanism, it will need to retrieve and serialize all data required by worker.
• documentation

[peterschutt]

Profiling here: https://gist.github.com/peterschutt/7530bc939ff5c268b2b8139e7d74596e

I'm not going to use pickle, artifact is 3x size of using json and speedup per serialize/deserialize iteration was minimal.