clarkmcc has created PR #1101: Fix #1099 and allow `ClientSession` to be used without moving in mongo-rust-driver

Issue Text:
This is a breaking change, though in most cases I suspect it won't actually break code unless people are truly passing non-`ClientSession` types that are `Into<&mut ClientSession>`. None of the tests in the repo were doing this that I could find.

In any case, not sure if this is a good idea, just want to get some eyes on the issue.