Use mongo-orchestration for testing

[kyeah]

Setting up multiple databases is tedious, especially when adding new version support. As @saghm mentions, we can do a lot better here.

[saghm]

Hello everyone! We've just released an alpha of an official MongoDB Rust driver; you can read about it here! Because of this, we won't be updating this driver anymore, and the repository will be made read-only as soon as I get finished posting these messages. Thanks for using this driver, and we hope you try out the new one!