Implement session

Implement the session, which will wrap the mongoc implementation. For every member function that talks to the server, add an overload with a client_session as the first parameter. List of files that contain these member functions:

• bulk_write.hpp
• change_stream.hpp
• client.hpp
• collection.hpp
• database.hpp
• index_view.hpp
• bucket.hpp
• downloader.hpp
• uploader.hpp