There is a potential memory leak in `Workerpool` due to the intricacies of
`time.Timer` stopping.

Whenever a `time.Timer` is `Stop`ped its channel must be cleared using a
`select` if the result of the `Stop()` is `false`.

Unfortunately in `Workerpool` these were checked the wrong way round.

However, there were a few other places that were not being checked.

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>