Add back commit groups

The commit group feature that allows multi-database transactions is removed from the latest version, because an elegant API design is not found. SQLite doesn't give us a way to associate a VFS with its database connection, so we had to use thread-local state. This causes confusion for the user, and does not work with languages that aggressively reschedules coroutines like Go.

No files, tests, or entry points are named. Start by reviewing the current transaction and VFS handling, then determine an API that restores multi-database commit groups without thread-local state and remains reliable when Go reschedules coroutines; done means those transactions work without the stated confusion or scheduling failure.

Written by the indexing model from the issue text.