To make binding into various languages easier, we should make some of our structures ref counted.

Primarily, this should help manage lifetime of structures which is much more difficult in higher-level languages where we cannot control scope.

In particular, the following are probably strong candidates for reference counting.

mongoc_cilent_t
mongoc_collection_t
mongoc_cursor_t
mongoc_database_t
mongoc_gridfs_t
mongoc_bulk_operation_t
mongoc_stream_t

Possibly these as well:

mongoc_uri_t
mongoc_write_concern_t
mongoc_read_preferences_t