vswitchd: Deprecate packet buffering in OVS 2.6.

vswitchd: Deprecate packet buffering in OVS 2.6.

OVS implementation of buffering packets that are sent to the
controller is not compliant with the OpenFlow specifications after
OpenFlow 1.0.  OVS implementation executes the buffered packet against
the actions of the modified or added rule, whereas OpenFlow (since
1.1) specifies that the packet should be matched against the flow
table 0 and processed accordingly.

Rather than fix this behavior, and potentially break OVS users, we
propose to remove the feature altogether, starting in OVS 2.7.  This
patch announces this in 'NEWS' for OVS 2.6, and adds detail to the FAQ
question about backet buffering.

Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>