net/http: release callbacks after fetch promise completes
    
    When the request context was canceled, the Transport.RoundTrip method
    could return before the fetch promise resolved. This would cause the
    success and failure callback functions to get called after they've
    been released, which in turn prints a "call to released function"
    error to the console.
    
    Avoid that problem by releasing the callbacks after the fetch promise
    completes, by moving the release calls into the callbacks themselves.
    This way we can still return from the Transport.RoundTrip method as
    soon as the context is canceled, without waiting on the promise to
    resolve. If the AbortController is unavailable and it's not possible to
    abort the fetch operation, the promise may take a long time to resolve.
    
    For #38003.
    
    Change-Id: Ied1475e31dcba101b3326521b0cd653dbb345e1d
    Reviewed-on: https://go-review.googlesource.com/c/go/+/226204
    Reviewed-by: Johan Brandhorst <johan.brandhorst@gmail.com>
    Reviewed-by: Richard Musiol <neelance@gmail.com>