{{Kubelet sends the PullImage request without timeout, because the image size
is unknown and timeout is hard to defined. The pulling request might run
into 0B/s speed, if containerd can't receive any packet in that connection.
For this case, the containerd should cancel the PullImage request.

Although containerd provides ingester manager to track the progress of pulling
request, for example `ctr image pull` shows the console progress bar, it needs
more CPU resources to open/read the ingested files to get status.

In order to support progress timeout feature with lower overhead, this
patch uses http.RoundTripper wrapper to track active progress. That
wrapper will increase active-request number and return the
countingReadCloser wrapper for http.Response.Body. Each bytes-read
can be count and the active-request number will be descreased when the
countingReadCloser wrapper has been closed. For the progress tracker,
it can check the active-request number and bytes-read at intervals. If
there is no any progress, the progress tracker should cancel the
request.}}