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: a1ee076d5f4a3965afe43d0bc23096dfdc170448

tools/portforward/portforward_test.go

@@ -68,6 +68,9 @@ func (c *fakeConnection) CloseChan() <-chan bool {
 	return c.closeChan
 }
 
+func (c *fakeConnection) RemoveStreams(_ ...httpstream.Stream) {
+}
+
 func (c *fakeConnection) SetIdleTimeout(timeout time.Duration) {
 	// no-op
 }