Workaround libevent bug

Workaround libevent bug
https://sourceforge.net/tracker/index.php?func=detail&aid=3078187&group_id=50884&atid=461324

The problem is that bufferevent_disable() doesn't disable EV_WRITE
when 'connecting' flag is set.  However from evhttp_connection_reset()
we want to disable EV_WRITE for sure (we are closing the socket next).
So we add bufferevent_disable_hard(), which acts like
bufferevent_disable(), but resets 'connecting' flag before the call to
the actual handler.

TODO: bufferevent_disable_hard() shouldn't be public, remove it from
event2/bufferevent.h.