support cleaning up old releases

[fho]

support cleaning up old releases

- change the "db cleanup" command syntax to accept a positional argument
  to specify if releases or tasks should be deleted,
- rename the --task-runs-before parameter to --before
- implement ad DeletedReleases method for the psql storage client, that
  works analogous to DeletedTaskRuns

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add created_at timestamp to release table

This will allow to cleanup releases that are older then X.

For all existing releases the created_at timestamp is set to the current
timestamp.

[fho]

merged manually