Raise level of occuring cron events to INFO

[dnaber-de]

Right now, the CronDebugListener records every cron worker event with the DEBUG severity:

[2018-04-30 14:32:21] DEBUG.DEBUG: Cron action "wp_scheduled_auto_draft_delete" performed.

Monitoring the proper execution of cron events is a typical scope of logging especially on production systems. Therefore I would suggest to raise the severity level to INFO as it is a normal operation without significant condition. Normally the threshold for log records for production environments is set to something above DEBUG to reduce the noise but right now this removes also the cron event log completely.

For sake of consistency the listener class should be renamed too, but this is way more invasive as it is considered public API, I guess?