PLT-3105 Fixed bugs with FileInfos migration, including duplicate FileInfos being saved (#4134)

* Added a limit to GetByPath for the rare cases when two old files had the same path

* Fixed files still being displayed for deleted posts

* Added a lock to prevent migrateFilenamesToFileInfos from migrating multiple posts at once

webapp/utils/async_client.jsx

@@ -722,6 +722,7 @@ export function getFileInfosForPost(channelId, postId) {
         return;
     }
 
+    callTracker[callName] = utils.getTimestamp();
     Client.getFileInfosForPost(
         channelId,
         postId,