Delegate the purging of 6 month+ old metrics to the purger.

Fixes timeouts during server startup.

The existing purging works in rails console using an old migrated database:

VmdbMetric.count => 4696666
require 'workers/schedule_worker'
ScheduleWorker::Jobs.new.metric_purge_all_timer
simulate_queue_worker
VmdbMetric.count => 0

Fixes #1029