Add support for OpenFlow 1.5 statistics (OXS).

Add support for OpenFlow 1.5 statistics (OXS).

This patch provides implementation Existing flow entry statistics are
redefined as standard OXS(OpenFlow Extensible Statistics) fields for
displaying the arbitrary flow stats.

To support this implementation below messages are newly added

OFPRAW_OFPT15_FLOW_REMOVED,
OFPRAW_OFPST15_AGGREGATE_REQUEST,
OFPRAW_OFPST15_FLOW_REPLY,
OFPRAW_OFPST15_AGGREGATE_REPLY,

The current commit adds support for the new feature in flow statistics
multipart messages, aggregate multipart messages and OXS support for flow
removal message, individual flow description messages.

Signed-off-by: Satya Valli <satyavalli.rama@tcs.com>
Co-authored-by: Lavanya Harivelam <harivelam.lavanya@tcs.com>
Signed-off-by: Lavanya Harivelam <harivelam.lavanya@tcs.com>
Co-authored-by: Surya Muttamsetty <muttamsetty.surya@tcs.com>
Signed-off-by: Surya Muttamsetty <muttamsetty.surya@tcs.com>
Co-authored-by: Manasa Cherukupally <manasa.cherukupally@tcs.com>
Signed-off-by: Manasa Cherukupally <manasa.cherukupally@tcs.com>
Co-authored-by: Pavani Panthagada <p.pavani1@tcs.com>
Signed-off-by: Pavani Panthagada <p.pavani1@tcs.com>
[blp@ovn.org simplified and rewrote much of the code]
Co-authored-by: Ben Pfaff <blp@ovn.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>