fix(sync/lmdb): construct right pk string for entity with multiple pr…

…imary keys

For the entity with multiple primary keys, like consumer_group_consumers, the
pk_string() function construct incorrect pk string like
`table: 0x028a49eda0:table: 0x0289c257b8`. Because it does not extract the
internal id value. Here, we fix it with original lmdb logic
get_cache_key_value(). Note this method directly gets id field name.

kong/db/schema/others/declarative_config.lua

@@ -60,9 +60,18 @@ do
     end
 
     CACHED_OUT:clear()
+
+    -- The logic comes from get_cache_key_value(), which uses `id` directly to
+    -- extract foreign key.
     for i = 1, count do
       local k = primary_key[i]
-      insert(CACHED_OUT, tostring(object[k]))
+      local v = object[k]
+
+      if type(v) == "table" and schema.fields[k].type == "foreign" then
+        v = v.id
+      end
+
+      insert(CACHED_OUT, tostring(v or ""))
     end
 
     return concat(CACHED_OUT, ":")