[cron] Avoid sending delayed events in when handling takes too long

[maxekman]

Currently if handling takes longer than the cron interval the next event will be sent directly after completion, but all other missed events will be skipped. Add a default on the cron select to skip events during busy handling. See: https://github.com/looplab/eventhorizon/blob/master/eventhandler/cron/eventhandler.go#L61