ofproto, ofp-util: Begin disentangling packet-in wire format and handling.

ofproto, ofp-util: Begin disentangling packet-in wire format and handling.

struct ofputil_packet_in mixes data included in OpenFlow packet_in messages
with data that used internally by ofproto and connmgr to queue and route
packet_ins.  This commit begins disentangling these purposes by adding a
new struct ofproto_packet_in that wraps struct ofputil_packet_in.  Adding
this new level of indirection causes a lot of code churn, so this commit
mainly takes care of that to make the remaining changes easier to read.

This commit does move the list node used for queuing packet_ins into the
new wrapper structure.

Signed-off-by: Ben Pfaff <blp@nicira.com>