ofproto-dpif: Store relevant fields for wildcarding in facet.

ofproto-dpif: Store relevant fields for wildcarding in facet.

Dynamically determines the flow fields that were relevant in
processing flows based on the OpenFlow flow table and switch
configuration.  The immediate use for this functionality is to
cache action translations for similar flows in facets.  This yields
a roughly 80% improvement in flow set up rates for a complicated
flow table.

More importantly, these wildcards will be used to determine what to
wildcard for the forthcoming kernel wildcard (megaflow) patches
that will allow wildcarding in the kernel, which will provide
significant flow set up improvements.

The approach to tracking fields and caching action translations in
facets was based on an impressive prototype by Ethan Jackson.

Co-authored-by: Ethan Jackson <ethan@nicira.com>
Signed-off-by: Ethan Jackson <ethan@nicira.com>
Signed-off-by: Justin Pettit <jpettit@nicira.com>