Handle duplicate archive file names.

The time period of the archive file currently depends on submit or start
time and whether the purge period is in hours, days, or months.
Previously, if the archive file name already exists, we would overwrite
the old archive file with the assumption that these are duplicate
records being archived after an archive load. However, that could result
in lost records in a couple of ways:

  * If there were runaway jobs that were part of an old archive file's
  time period and are later fixed and then purged, the old file would
  be overwritten.
  * If jobs or steps are purged but there are still jobs or steps in
  that time period that are pending or running, the pending or running
  jobs and steps won't be purged. When they finish and are purged, the
  old file would be overwritten.

Instead of overwriting the old file, we append a number to the file name
to create a new file. This will also be important in an upcoming commit.

Bug 6033.