ofproto-dpif: Modularize ofproto-dpif-xlate.

ofproto-dpif: Modularize ofproto-dpif-xlate.

This patch modularizes ofproto-dpif-xlate by disentangling it from
ofproto-dpif.  Instead of poking around in ofproto-dpif's internal
data structures, ofproto-dpif-xlate is updated with a simple API
which can easily be made thread safe.  There are still some places
where ofproto-dpif-xlate needs to call into ofproto-dpif, but this
patch makes significant progress towards the final goal.

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