ensure the log weasel id is popped when invoking a job

in some circumstances the invoke_job callback is invoked twice for the same job. the first time around we pop the log weasel id arg and start the log weasel transaction. the second time the callback is invoke the plugin basically does nothing.

there's no good way to improve the tests to make assertions on the second invocation of the callback because dj doesn't support invoking a before callback without its corresponding after callback (which in the case of the log weasel plugin destroys the log weasel transaction).

this should address the issue of `Delayed::Worker.work_off` causing that relay test to fail.