transaction: write pending generated files Such file are generated with a .pending prefix. It is up to the reader to implement the necessary logic for reading pending files. We add a test to ensure pending files are properly cleaned-up in both success and error cases.

transaction: have _generatefile return a boolean The function returns True if any files were generated. This will be used to know if any pending files have been written.

transaction: allow generating files with a suffix This will allow us to generate temporary pending files. Files generated with a suffix are assumed temporary and will be cleaned up at the end of the transaction.

transaction: fix some docstring grammar