Add test and fix for nested savepoint rollbacks.
Add test and fix for nested savepoint rollbacks.

This requires giving savepoints distinct names.

jgallagher/rusqlite,jgallagher/rusqlite,mikelodder7/rusqlcipher,xinghun92/rusqlite,gwenn/rusqlite,mikelodder7/rusqlcipher,xinghun92/rusqlite,gwenn/rusqlite,jgallagher/rusqlite,gwenn/rusqlite,mikelodder7/rusqlcipher,xinghun92/rusqlite