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.
remove redundant assignment
Merge pull request #4 from newcolabs/work-off

ensure the log weasel id is popped when invoking a job