CXX-1068 Fix incomplete bsoncxx iterator API

This commit rationalizes the bsoncxx iterator API for both
bsoncxx::document::view and bsoncxx::array::view:

* The non-const iterator implementation is removed; for both document::view
  and array::view, iterator is a type alias to const_iterator.
- begin(), cbegin(), end(), cend(), and find() always return
  a const_iterator.

This replaces and closes #555.