refactor(collection): move floating functions to collection_ops

* refactor(collection): move floating functions to collection_ops

Fixes NODE-1255

* use collection_ops instead of collection

* incorporate feedback and document operations

* incorporate feedback

* move checkForAtomicOperators to collection_ops

* move countDocuments to collection_ops

* move decorateWithReadConcern to utils

* include decorateWithReadConcern properly

* fix createIndexExample3 test

* incorporate feedback

Author: kvwalker