Refactor: use "Document" suffix for classes that inherit from mongoengine's Document
ie.

- `Job` -> `JobDocument`
- `CachedResponse` -> `CachedResponseDocument`
- `JobTotalMetric` -> `JobTotalMetricDocument`
- `CacheTotalMetric` -> `CacheTotalMetricDocument`

The idea is to clarify that the class allows manipulating the DB directly and to let the non-suffixed names be free for use as a more high-level class.

It's a nit - easy and useful, but not urgent
Refactor: use "Document" suffix for classes that inherit from mongoengine's Document: ie.

- `Job` -> `JobDocument`
- `CachedResponse` -> `CachedResponseDocument`
- `JobTotalMetric` -> `JobTotalMetricDocument`
- `CacheTotalMetric` -> `CacheTotalMetricDocument`

The idea is to clarify that the class allows manipulating the DB directly and to let the non-suffixed names be free for use as a more high-level class.

It's a nit - easy and useful, but not urgent