Add tests for current filter types and refactor filter creation

[desean1625]

There will need to be some manual testing but I think I got all the various filters and test added before the refactor.

Need to double check filters created from controls I think that is what this code block was from

                if filter_key == "query":
                    filt_index = list(f.get(filter_key))[0]
                    filter_dict["must_not"].append({filt_index: f.get(filter_key).get(filt_index)})

when the object nested under query wasn't bool and the key wasn't provided

[desean1625]

@natebynum20 This is ready for review and merge. It fixes three issues.

1. not all filter cases where being parsed correctly
2. if a bad filter was sent like

   {
   "exits": {
   "field": "name"
   }
   }

   the datashader would immediately have the client try again causing valid requests to get interrupted when gunicorn shuts down the worker after x number of requests to clean up memory leaks.

3. the cleanup cache code wasn't cleaning up empty directories this would lead to a build up of directories that would eventually cause glob to take longer than the default gunicorn timeout eventually putting datashader in a state where it wouldn't start up (this would take a month or two)

[BenShoeSpectric]

Nate, Did you have a chance to double check the commits after you approval?

[natebynum20]

Just went over the commits that were submitted after my initial approval. Looks good from my stand point