A new `boulder-janitor` command is added that provides a long-running
daemon that cleans up rows associated with expired certificate
resources. At present this is rows from the following tables:
* certificates
* certificateStatus
* certificatesPerName
Adding cleanup of tables associated with Order resources is the next step.
Three prometheus stats are exported:
* janitor_deletions - CounterVec for the number of deletions by table the
boulder-janitor has performed.
* janitor_workbatch - GaugeVec for the number of items of work by table
the boulder-janitor queued for deletion.
* janitor_errors - CounterVec for the number of errors by table and error
type the boulder-janitor has experienced.
9e896325f7