ofproto: Optimise OpenFlow flow expiry

ofproto: Optimise OpenFlow flow expiry

Optimise OpenFlow flow expiry by placing expirable flows on a list.
This optimises scanning of flows for expiry in two ways:

* Flows that will never expire are not traversed.

  This addresses a case seen in the field.  With 1,000,000 flows that
  are not expirable, this dramatically reduces CPU utilization to
  approximately zero.

* Empirically list traversal appears faster than the code it replaces.

  With 1,000,000 expirable flows present an otherwise idle system I
  observed CPU utilisation of around 20% with the existing code but
  around 10% with this new code.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>