Majority of scheduled tasks DB records are completed tasks.
We check completed tasks only for Migration worker task type.
Excluding other workers' completed tasks significantly speeds up the query
on sites with longer history. E.g. on mailpoet.com the query sped up from 0.6s to 0.003s.
[MAILPOET-4314]
By default checkRunInterval will always return true because there is no way
the time which has passed since the last run is smaller than -1. To prevent
unnecessary code execution and an not needed SQL insert, we can return true
in checkRunInterval() early.
[MAILPOET-4267]
