Add document_count support in Collection

[tristandeborde]

Hello,

I have noticed that pymongo version 3.7.2 logs warnings when the collection.count() method is called:

count is deprecated. Use Collection.count_documents instead.
The topic is discussed in more details in this stackoverflow thread and in this other thread as well.

To solve the problem, I have just added a count_collection() and an estimated_document_count() method to mongokat's collection class, wrapping around the pymongo methods.
Would you be okay with merging these small changes ?

Thanks !