of: set TCP_NODELAY for secure channel socket

Due to Nagle, an OF message that an application sends might be not
delivered to a switch immediately. This patch disables Nagle. This has
bad effect on throughput. I think that the latency matters for any
management channel. To avoid throughput drop, we could use TCP_CORK on
Linux but it's linux specific.

Reviewed-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
RyuApp: add methods to listen/unlisten specific event

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
RyuApp: allow listening events without having handlers

this will be used by ofctl app which registers handlers dynamically.

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
ofp_event: add a function to find Event class for given OFP class

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
ryu.app.ofctl: implement reception of reply message

an example:
    msg = parser.OFPEchoRequest(datapath=datapath)
    result = ofctl.api.send_msg(self, msg, reply_cls=parser.OFPEchoReply)
    assert isinstance(result, parser.OFPEchoReply)

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
ryu.app.ofctl: implement multipart

an example:
    msg = parser.OFPPortDescStatsRequest(datapath=datapath)
    result = ofctl.api.send_msg(self, msg,
                                reply_cls=parser.OFPPortDescStatsReply,
                                reply_multi=True)

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
ryu.app.ofctl: raise exceptions on openflow error messages

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>