ovn-controller: Change strategy for gateway conntrack zone allocation.

ovn-controller: Change strategy for gateway conntrack zone allocation.

Commit 263064aeaa31e7 (Convert binding_run to incremental processing.)
changed the way patched_datapaths were handled. Previously we would
destroy the datastructure in every run and re-create it fresh. The new
way causes problems with the way conntrack zones are allocated as now
we can have stale port_binding entries causing segmentation faults.

With this commit, we simply don't depend on port_binding records in
conntrack zone allocation and instead store the UUID as a string in
the patch_datapath datastructure.

(The test enhanced with this commit would fail without the changes
in the commit. i.e. ovn-controller would crash. )

Signed-off-by: Gurucharan Shetty <guru@ovn.org>
Acked-by: Ryan Moats <rmoats@us.ibm.com>