Prevent timer leaks in Workerpool and others

[zeripath]

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

Whenever a time.Timer is Stopped 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

[6543]

🚀

[kevans91]

I suspect some of these drains might be able to go away to simplify things -- according to https://sourcegraph.com/github.com/golang/go/-/commit/ba3149612f62c011765876d7a437095fa50e0771 and some clarification on IRC, draining the channel isn't to prevent a leak within the channel but to attempt prevention of a false-positive on a consumer of the timer. So the two situations you'd actually be required to drain it are:

1.) If you're going to subsequently Reset() the timer, or
2.) If anyone waiting on the channel might be sensitive to the timer firing just before you stopped it

I think the majority of these consumers fall into neither, as the timer's oftentimes only consumed by the single channel and not getting re-used.

[zeripath]

make lg-tm work