Split up UploadArea

[sampierson]

CodeClimate is complaining that

File upload_area.py has 253 lines of code (exceeds 250 allowed). Consider refactoring.

Looking at UploadArea, there are a couple things that could be broken out of it to simplify it:

1. It knows a lot about SQS. Perhaps we should encapsulate all our SQS knowledge inside a class called, say. SQSQueue which has subclasses CsumSQSQueue and DeleteSQSQueue, and from UploadArea be simply calling DeletionSQSQueue().enqueue(upload_area).
2. Methods retrieve_file_checksum_statuses_for_upload_area and retrieve_file_validation_statuses_for_upload_area don't act on the current object except to get a database handle from self.db. This diagnostic / health check logic could live in a separate module.