1. Checkpoint.undo may be after checkpoint itself:

1. Checkpoint.undo may be after checkpoint itself:
   - no more elog(STOP) in StartupXLOG();
   - both checkpoint' undo & redo are used to define
     oldest on-line log file.
2. Ability to pre-allocate a few log files at checkpoint time
   (wal_files option). Off by default.