Correct port-forward data copying logic so that the server closes its
half of the data stream when socat exits, and the client closes its half
of the data stream when it finishes writing.

Modify the client to wait for both copies (client->server,
server->client) to finish before it unblocks.

Fix race condition in the Kubelet's handling of incoming port forward
streams. Have the client generate a connectionID header to be used to
associate the error and data streams for a single connection, instead of
assuming that streams n and n+1 go together. Attempt to generate a
pseudo connectionID in the server in the event the connectionID header
isn't present (older clients); this is a best-effort approach that only
really works with 1 connection at a time, whereas multiple concurrent
connections will only work reliably with a newer client that is
generating connectionID.