ovn-northd: Introduce pre-stateful table.

ovn-northd: Introduce pre-stateful table.

Currently, the only use of stateful services in conntrack is
OVN ACLs. In table pre-ACL, we send the packet to conntrack
to track it (to get its status) and to defrag via the ct_next
action.

As we introduce more stateful services, the ACL feature will
have to share the conntrack module with others. As
preparation for more stateful features like loadbalancing,
this commit introduces a new pre-stateful table that is
responsible to send packets through conntrack via
ct_next action. If pre-ACL table needs to send a packet
through conntrack, it just sets the 'reg0[0]' as 1.
Pre-stateful table in-turn will send the packet to conntrack
if 'reg0[0]' is 1.

Signed-off-by: Gurucharan Shetty <guru@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>