v1: transaction (CXX-3237, CXX-3238)

[eramongodb]

Resolves CXX-3237 and CXX-3238 for the v1::transaction component.

Unlike recently-reviewed PRs, the v_noabi::options::transaction class does not contain T const& getters, thus it can be fully reimplemented in terms of v1::transaction. However, unlike previously-reviewed PRs for classes which were reimplemented in terms of v1, this is the first class which may throw an "invalid object" exception for the assign-or-destroy-only state. In order to preserve v_noabi API backward compatibility while respecting the v1 API's own assign-or-destroy-only invariants, the check_moved_from() helper implements the null check by reusing the (unchecked) raw pointer returned by the v1 as_mongoc() internal helper function. This pattern will also used by upcoming components/PRs for other classes which may throw an "invalid object" exception for backward compatibility (that is, classes which have a potentially-throwing _get_impl() helper).