We now run a culling service once a day to find all unpacked tar ball
directories whose age is beyond the configured maximum.  We also have
changed the unpacked tar balls service to only unpack tar balls less
than the configured maximum age.

One can keep an already unpacked tar ball by creating a `.__pbench_keep__`
file in the top level of the unpacked tar ball directory.

A report is generated much like the following:

```
Culled 3 unpacked tar ball directories (0 errors) in 0.00 secs

Actions Taken:
  - controller-no-prefixes/tarball_culled_1970.01.01T00.00.00 (0 errors, 0.00 secs)
      $ rm results/controller-no-prefixes/tarball_culled_1970.01.01T00.00.00  # succ
      $ mv incoming/controller-no-prefixes/tarball_culled_1970.01.01T00.00.00 incoming/controller-no-prefixes/.delete.tarball_culled_1970.01.01T00.00.00  # succ
      $ rmtree incoming/controller-no-prefixes/.delete.tarball_culled_1970.01.01T00.00.00  # succ
  - controller-prefixes/tarball_culled-w-prefix_1970.01.01T00.00.00 (0 errors, 0.00 secs)
      $ rm results/controller-prefixes/pre0/pre1/pre2/tarball_culled-w-prefix_1970.01.01T00.00.00  # succ
      $ mv incoming/controller-prefixes/tarball_culled-w-prefix_1970.01.01T00.00.00 incoming/controller-prefixes/.delete.tarball_culled-w-prefix_1970.01.01T00.00.00  # succ
      $ rmtree incoming/controller-prefixes/.delete.tarball_culled-w-prefix_1970.01.01T00.00.00  # succ
  - controller-prefixes/tarball_culled-w-userA_1970.01.01T00.00.00 (0 errors, 0.00 secs)
      $ rm results/controller-prefixes/path0/path1/tarball_culled-w-userA_1970.01.01T00.00.00  # succ
      $ rm users/userA/controller-prefixes/path0/path1/tarball_culled-w-userA_1970.01.01T00.00.00  # succ
      $ mv incoming/controller-prefixes/tarball_culled-w-userA_1970.01.01T00.00.00 incoming/controller-prefixes/.delete.tarball_culled-w-userA_1970.01.01T00.00.00  # succ
      $ rmtree incoming/controller-prefixes/.delete.tarball_culled-w-userA_1970.01.01T00.00.00  # succ
```