dpset: prefix private methods with _

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

this "dp_type" stuff has been there for almost 2 years
without being used at all.

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

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

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

this should fix the following crash recently reported
by wataru yamamoto on ryu-devel.

hub: uncaught exception: Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/ryu/lib/hub.py", line 48, in _launch
    func(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/ryu/base/app_manager.py", line 110, in _event_loop
    handler(ev)
  File "/usr/local/lib/python2.7/dist-packages/ryu/controller/dpset.py", line 157, in dispacher_change
    self.register(datapath)
  File "/usr/local/lib/python2.7/dist-packages/ryu/controller/dpset.py", line 100, in register
    assert dp.id not in self.dps
AssertionError

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