SqliteVtabNestedTransactionCallback
This method provide the virtual table implementation an opportunity to implement nested transactions.
When xSavepoint(X,N) is invoked, that is a signal to the virtual table X that it should save its current state as savepoint N. A subsequent call to xRollbackTo(X,R) means that the state of the virtual table should return to what it was when xSavepoint(X,R) was last called. The call to xRollbackTo(X,R) will invalidate all savepoints with N>R; none of the invalided savepoints will be rolled back or released without first being reinitialized by a call to xSavepoint(). A call to xRelease(X,M) invalidates all savepoints where N>=M.