loader: Use new configuration stanza to filter and buffer data

Related T1389

conf/loader.yml

 storage:
-  cls: remote
+  cls: filter
   args:
-    url: http://swh-storage:5002/
+    storage:
+      cls: buffer
+      args:
+        storage:
+          cls: remote
+          args:
+            url: http://swh-storage:5002/
+        min_batch_size:
+          content: 10000
+          content_bytes: 104857600
+          directory: 1000
+          revision: 1000
+
 celery:
   task_broker: amqp://guest:guest@amqp//
   task_modules:

[Phabricator Migration user]

Some references in the commit message have been migrated:

• T1389 is now swh/meta#1389 (closed)