ofproto: Eliminate 'ofproto_node' member from struct rule.

ofproto: Eliminate 'ofproto_node' member from struct rule.

The ofproto_node member is convenient for collecting lists of rules, but
it is also challenging for concurrency because only a single thread at a
time can put a given rule on a list.  This commit eliminates the
ofproto_node member and introduces a new 'struct rule_collection' that
can be use in a thread-safe manner.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>