ofproto-dpif: Scope revalidation state to dpif_backers.

Before this patch, the "need_revalidate" flag and the
"revalidate_set" tag_set where maintained separately for each
ofproto.  This won't work in future patches when a flow table
change in one ofproto can require revalidation in another entirely
separate ofproto.  For this reason, this patch scopes these flags
to the dpif_backers.

Signed-off-by: Ethan Jackson <ethan@nicira.com>