use parallel lookups for lg/routers

inventory_provider/routes/lg.py

 import json
 
-from flask import Blueprint, jsonify, Response
+from flask import Blueprint, jsonify, Response, current_app
 
 from inventory_provider.routes import common
 
@@ -25,29 +25,25 @@ def routers(access):
             status=404,
             mimetype='text/html')
 
-    redis = common.get_current_redis()
-
     def _visible(router):
         if access == ACCESS_INTERNAL:
             return True
         return router['type'] == 'CORE'
 
     def _routers():
-        i = 0
-        for k in redis.scan_iter('opsdb:lg:*'):
-            rtr = redis.get(k.decode('utf-8')).decode('utf-8')
-            rtr = json.loads(rtr)
-            i += 1
-            if _visible(rtr):
-                yield rtr
+        for doc in common.load_json_docs(
+                config_params=current_app.config['INVENTORY_PROVIDER_CONFIG'],
+                key_pattern='opsdb:lg:*'):
+            yield doc['value']
 
+    redis = common.get_current_redis()
     cache_key = f'classifier-cache:lg:{access}'
     result = redis.get(cache_key)
 
     if result:
         result = json.loads(result.decode('utf-8'))
     else:
-        result = list(_routers())
+        result = list(filter(_visible, _routers()))
         # cache this data for the next call
         redis.set(cache_key, json.dumps(result).encode('utf-8'))