Backport #22146
There are a few places in FlushQueueWithContext which make an incorrect
assumption about how `select` on multiple channels works.
The problem is best expressed by looking at the following example:
```go
package main
import "fmt"
func main() {
closedChan := make(chan struct{})
close(closedChan)
toClose := make(chan struct{})
count := 0
for {
select {
case <-closedChan:
count++
fmt.Println(count)
if count == 2 {
close(toClose)
}
case <-toClose:
return
}
}
}
```
This PR double-checks that the contexts are closed outside of checking
if there is data in the dataChan. It also rationalises the WorkerPool
FlushWithContext because the previous implementation failed to handle
pausing correctly. This will probably fix the underlying problem in
#22145Fix#22145
Signed-off-by: Andrew Thornton <art27@cantab.net>
3ad62127df