make sure that xids_owner_xid_key only gets created if it doesn't exist

would lead to errors when running the add_xid_constraint migration on properly set up db; migration should be idempotent

server/postgres/migrations/000002_add_xid_constraint.sql

@@ -1,3 +1,14 @@
 ALTER TABLE xids
-  DROP CONSTRAINT IF EXISTS xids_owner_uid_key,
-  ADD CONSTRAINT xids_owner_xid_key UNIQUE (owner, xid);
+  DROP CONSTRAINT IF EXISTS xids_owner_uid_key;
+
+DO $$
+BEGIN
+    IF NOT EXISTS ( SELECT  constraint_schema
+                ,       constraint_name
+                FROM    information_schema.constraint_column_usage
+                WHERE   constraint_name = 'xids_owner_xid_key'
+              )
+    THEN
+        ALTER TABLE xids ADD CONSTRAINT xids_owner_xid_key UNIQUE (owner, xid);
+    END IF;
+END$$;