Task executions that are terminated by a fatal error may never be executed again. #48

Hi there,
I'm noticing that when a task is terminated during execution due to a PHP fatal error (such as memory exhausted), it retains the 'running' status.
As a result, a recurring task is never scheduled again:

For task handlers that implement the LockingTaskHandlerInterface interface, we may use the lock's ttl to exclude expired running task executions.