Improve context cancelation

[pellared]

In lot of places the following pattern is used for cancelation:

for {
    select {
    case <-ctx.Done():
        return
    case x := <- ch:
        foo(x)
    }
}

The problem is that there select does not guarantee the order when both channels signal. Therefore if ch will have multiple values buffered then the code block would eventually return instead of returning as soon as possible. The easy way to fix it is to use the following pattern:

for {
    if err := ctx.Err(); err != nil {
        return
    }
    select {
    case <-ctx.Done():
        return
    case x := <- ch:
        foo(x)
    }
}

or

for {
    select {
    case <-ctx.Done():
        return
    default:
    }
    select {
    case <-ctx.Done():
        return
    case x := <- ch:
        foo(x)
    }
}