Move indexer attributes to a query

[marceloboeira]

Right now the indexer "query" is done on every request... it would be better to have it as a query...

Instead of

Episode.all.each do |episode|

if episde....

end

Episode.not_indexed.each do |episode|
...
end

where Episode.not_indexed is:

class Episode
  scope :not_indexed -> { where(indexed: nil).or(....) }
end
```

That improves the performance because it does not fetch all episodes from mongo to ruby's memory so it can run against the if-else.

(Add a mongo-index to `updated_at` && `indexed_at` so it can run faster)

[marceloboeira]

related to #13