Maintain separate async and sync connections to nl:0 in secchan.

Maintain separate async and sync connections to nl:0 in secchan.

When a network link is flooded with traffic, secchan's OpenFlow socket
queue becomes congested with traffic.  This leaves no room for replies
to OpenFlow requests relayed to that socket by secchan.

This commit modifies secchan to use separate sockets for asynchronous
traffic and for OpenFlow requests/replies, which should avoid the problem.