Improve usage of defaultworkertimeout

[michielkeijts]

See #400 for explanation.

• Fixes error in not using a relative DateTime for finding running jobs which are out of time
• Throw exception for usage of defaultworkertimeout of 0 seconds as it is clearly not supported.

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (82bed55) 72.53% compared to head (fea8279) 72.52%. Report is 1 commits behind head on master.

Files	Patch %	Lines
src/Queue/Config.php	75.00%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #401 +/- ## ============================================ - Coverage 72.53% 72.52% -0.01% - Complexity 684 685 +1 ============================================ Files 40 40 Lines 2450 2453 +3 ============================================ + Hits 1777 1779 +2 - Misses 673 674 +1 ```

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