Fix DB performance with sessions

Commit 9d15b98261 added a method to
get objects from the DB (OVO or otherwise). This introduced a
performance problem, as seen when running "neutron port-list" with
a large number of neutron ports. On one setup with 6.5k ports, the
port-list time was on the order of several minutes. This patch makes
a minor change to this method, and imports sqlalchemy instead of just
the inspect method from that library. This reduced the same port-list
time to a low double-digit number of seconds. This follows the same
pattern that was introduced in upstream neutron (see this patch:
https://review.opendev.org/c/openstack/neutron/+/428779).

Change-Id: Id43811d2526566a466747d7e94bde8fd82430355

gbpservice/neutron/db/api.py

@@ -20,7 +20,7 @@
 # corresponding to the newest neutron branch supported by this
 # repository.
 
-from sqlalchemy import inspect
+import sqlalchemy
 
 import neutron.objects.base as n_base
 from neutron_lib.db import api
@@ -48,7 +48,7 @@ def get_session_from_obj(db_obj):
     if isinstance(db_obj, n_base.NeutronObject):
         return db_obj.obj_context.session
     try:
-        instance = inspect(db_obj)
+        instance = sqlalchemy.inspect(db_obj)
         return instance.session
     except Exception:
         return None