Transactions support

Engineer(s): Kaitlin, Patrick 2020-05-15: Updated target of 2020-05-29 Adding 2 more weeks, but we'll likely wrap this up in the next week or so Implement convenient API transactions in progress Support sharding tests in transactions spec test runner in code review 2020-04-30: Updated target of 2020-05-15 Patrick halted work on this while focusing on Rust sessions, should have about a week's worth left once back working on this 2020-04-17: Updated target of 2020-05-01 Patrick has picked up some of this work since Jamie's rotation ended Implement convenient API, support sharding tests in transactions spec test runner in progress Transactions spec test runner, Update transactions guides/error handling merged 2020-04-03: On track Design for Swift driver transactions complete Transaction spec test runner in progress 2020-03-20: Initial end date of 2020-04-10 (4 weeks) Jamie is working on the design for Swift driver transactions