ofproto-dpif: Implement self-check of flow translations.

ofproto-dpif: Implement self-check of flow translations.

One of the major tasks of ofproto-dpif is to translate OpenFlow
actions into "ODP" datapath actions.  These translations are essentially
a cache that requires revalidation when certain state changes occur.  For
best performance it's important to revalidate flows only when necessary,
so from time to time Open vSwitch has gotten this wrong, which meant that
stale flows could persist in the kernel and cause surprising behavior.

This commit implements a simple "self check": every trip through the
Open vSwitch main loop randomly chooses one flow entry and checks that
its actions have been correctly translated.  If not, Open vSwitch logs
the details of the problem.  This should help find problems more
quickly in the future.

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