Revives #184.
The rewrite for async/await and Tokio accidentally lost functionality
that allowed users to assign specific names to savepoints when using
nested transactions. This functionality had originally been added
in #184 and had been updated in #374.
This commit revives this functionality using a similar scheme to the
one that existed before. This should allow CockroachDB users to update
to the next patch release of version `0.17`.
64d6e97eff