ofproto-dpif-xlate: Take responsibility for ofproto_receive().

ofproto-dpif-xlate: Take responsibility for ofproto_receive().

ofproto_receive() is a slightly odd function which doesn't fit
perfectly in either ofproto-dpif or ofproto-dpif-xlate.  However, it's
much easier to reason about its thread safety in ofproto-dpif-xlate,
so this patch moves it there and renames it xlate_receive().

Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>