ofproto-dpif-xlate: Implement RCU locking in ofproto-dpif-xlate.

Before, a global read-write lock protected the ofproto-dpif
/ ofproto-dpif-xlate interface.  Handler and revalidator threads
had to wait while configuration was being changed.  This patch
implements RCU locking which allows handlers and revalidators
to operate while configuration is being updated.

Signed-off-by: Ryan Wilson <wryan@nicira.com>
Acked-by: Alex Wang <alexw@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>
Signed-off-by: Alex Wang <alexw@nicira.com>