Remove using and forward-declarations from collection.hpp

From CR
https://mongodbcr.appspot.com/188860002/diff/1/src/mongocxx/collection.hpp#newcode66

At head of file:

• Remove using typedefs (using bsoncxx::builder::basic::make_document etc)
• Ensure class client and class database need to be forward-declared; remove if not. If included header shouldn't be necessary.