vconn-stream: Always call the stream's run function from vconn_stream_run().

The stream's run function ensures that data buffered in the stream itself
gets pushed to the network.  Only the SSL stream class has such a run
function, which means that SSL stream data failed to be pushed to the
remote peer in a timely manner in some cases.

Many thanks to Alex Yip for narrowing this down.

Reported-by: Alex Yip <alex@nicira.com>
Tested-by: Alex Yip <alex@nicira.com>
Bug #6221.