Cleanup portforward streams after their usage

This implements a stream cleanup when using portforwardings. Before
applying this patch, the streams []httpstream.Stream within
`spdy/connection.go` would fill-up for each streaming request. This
could result in heavy memory usage. Now we use the stream identifier to
keep track of them and finally remove them again once they're no longer
needed.

Signed-off-by: Sascha Grunert <sgrunert@redhat.com>

Kubernetes-commit: b14bd44f33d93e1ee64c1d68fa7591d79eac5893

tools/portforward/portforward_test.go

@@ -69,6 +69,9 @@ func (c *fakeConnection) CloseChan() <-chan bool {
 	return c.closeChan
 }
 
+func (c *fakeConnection) RemoveStreams(_ ...httpstream.Stream) {
+}
+
 func (c *fakeConnection) SetIdleTimeout(timeout time.Duration) {
 	// no-op
 }