ofproto-dpif: Implement PACKET_IN in userspace.

ofproto-dpif: Implement PACKET_IN in userspace.

In future patches, PACKET_IN messages will include meta-data which
is only available in userspace during action translation.  Either,
this data needs to be stored until it's required by a userspace
datapath action, or the PACKET_IN messages must be sent at the time
the data is available.  This patch implements the latter.

Signed-off-by: Ethan Jackson <ethan@nicira.com>