Filter out finished statuses in the BaseConsumer

[willgraf]

In ImageFileConsumer._consume we check that the hash's status is not a finished status. This seems pretty useful, and should probably be moved into the BaseConsumer class, perhaps in get_redis_hash. That way, future custom consumers can do not need to implement it themselves.

Originally posted by @willgraf in #115

[willgraf]

After more consideration, I'm not sure how useful this would be.

Yes, the BaseConsumer can check the status of the hash before we pass it to _consume. But we will still have a _consume function defined that takes in a Redish hash and only needs to do work if the hash's status is valid. It seems like that logic should be incorporated into _consume, and not into whatever function calls _consume.

Even if we baked the log into the parent class, new consumers would be required to call super, which is a bit less clear about what's going on.