Improved Versioning Support (#598)

Versions are now recorded in the database. New public API methods for
managing versions:

- `DBOS.list_application_versions()` — Returns all versions, newest
first
  - `DBOS.get_latest_application_version()` — Returns the latest version
- `DBOS.set_latest_application_version(version_name)` — Sets a version
as latest by updating its timestamp to now

Related changes:
- A DBOS executor not of the latest version will print a warning on
startup.
- Scheduled workflows are now always enqueued to the latest version.

Also improve support for scheduling workflows in classes.