Serialization through util.inspect always occurs, regardless of debug setting #2

As reported here, util.inspect serialization occurs regardless of whether debugging is enabled, and this can cause memory usage issues with large objects.

The debugging logic needs to be changed to use a custom formatter instead, so that the serialization logic is only executed when debugging is enabled.

As reported [here](https://github.com/joepie91/node-promise-task-queue/pull/3#issuecomment-274944105), `util.inspect` serialization occurs regardless of whether debugging is enabled, and this can cause memory usage issues with large objects. The debugging logic needs to be changed to use a custom formatter instead, so that the serialization logic is only executed when debugging is enabled.

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Serialization through util.inspect always occurs, regardless of debug setting #2