ofproto: update flow_stats flags on flow_stats_request

ofproto: update flow_stats flags on flow_stats_request

This is a first step in implementing 'on demand flow counters'.

We save relevant flow_mod flags (OFPUTIL_FF_SEND_FLOW_REM,
OFPUTIL_FF_NO_PKT_COUNTS, OFPUTIL_FF_NO_BYT_COUNTS) into newly created rule
when a new flow is added, and echo them back in the flow stats request.

Notice that we remove send_flow_removed flag from struct rule/struct ofoperation
and we replace it with an enum tracking all ofputil_flow_mod_flags.

Signed-off-by: Daniel Baluta <dbaluta@ixiacom.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>