Queue#pause waits until active jobs are finished

Created by: xdc0

This PR makes Queue#pause wait until all jobs in active state are completed before resolving.

Previously, when pausing a queue, it would immediately resolve the pause promise, making it difficult to perform a clean disconnection. Queue#pause will consume the private method Queue#whenCurrentJobsFinished, which checks jobs in active state, if there are pending jobs, then it sets up a listener on the completed event to recheck for jobs in active state. Once active jobs reaches 0, it resolves and in turns resolves the promise returned by Queue#pause