ofproto-dpif: Implement "flow setup governor" to speed up many short flows.

ofproto-dpif: Implement "flow setup governor" to speed up many short flows.

The cost of creating and initializing facets and subfacets and installing,
tracking, and uninstalling kernel flows is significant.  When most flows
have only one or a few packets, this overhead is higher than the cost of
handling each packet individually.  This commit introduces heuristics that
cheaply count (approximately) the number of packets seen in a flow and
skips most of this expensive bookkeeping until the packet count exceeds a
threshold (currently 5 packets).

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