Do not take into account old deferred tasks

A deferred task will remain deferred forever if the task it depends on
fails. When checking for waiting tasks, we should ignore deferred tasks
that are too old.
"Too old" is currently set to 30 minutes. We shouldn't have tasks that
take that long. We might want to shorten that even a bit (to not block
new identical tasks to be run) but if a task failed there is probably
something to be done.

JIRA INFRA-559 #action In Progress