Implement `count_documents()` and `estimated_document_count()`

[IlyaSkriblovsky]

Collection.count() and deprecated in PyMongo and replaced by two methods:

• estimated_document_count() that always metadata-only, may be inaccurate and does not support transactions, but is always lightning-fast
• count_documents() that uses a simple aggregation pipeline under the hood, always correct and supports transactions.

TxMongo should do the same.