Store last execution time

1. Why there's a check for ScheduledCronServiceInterface in getLastExecutionTime() ? As I understand the last time value is stored for every handler.
2. Technically, 0 is valid unix timestamp, so I'm not sure why getLastExecutionTime() shouldn't return null in case the value is missing.

I addressed the concerns in #4, and rebased against 2.0.x.

vasyl.kletsko → made their first commit to this issue’s fork.

Looks good now.

Automatically closed - issue fixed for 2 weeks with no activity.