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

i18n/en.json

@@ -885,10 +885,22 @@
     "id": "api.file.migrate_filenames_to_file_infos.file_not_found.warn",
     "translation": "Unable to find file when migrating post to use FileInfos, post_id=%v, filename=%v, path=%v, err=%v"
   },
+  {
+    "id": "api.file.migrate_filenames_to_file_infos.get_file_infos_again.warn",
+    "translation": "Unable to get FileInfos for post after migratio, post_id=%v, err=%v"
+  },
+  {
+    "id": "api.file.migrate_filenames_to_file_infos.get_post_again.warn",
+    "translation": "Unable to get post when migrating to use FileInfos, post_id=%v, err=%v"
+  },
   {
     "id": "api.file.migrate_filenames_to_file_infos.info.app_error",
     "translation": "Unable to fully decode file info when migrating post to use FileInfos, post_id=%v, filename=%v, err=%v"
   },
+  {
+    "id": "api.file.migrate_filenames_to_file_infos.migrating_post.debug",
+    "translation": "Migrating post to use FileInfos, post_id=%v, err=%v"
+  },
   {
     "id": "api.file.migrate_filenames_to_file_infos.mismatched_filename.warn",
     "translation": "Found an unusual filename when migrating post to use FileInfos, post_id=%v, channel_id=%v, user_id=%v, filename=%v"
@@ -897,9 +909,13 @@
     "id": "api.file.migrate_filenames_to_file_infos.no_filenames.warn",
     "translation": "Unable to migrate post to use FileInfos with an empty Filenames field, post_id=%v"
   },
+  {
+    "id": "api.file.migrate_filenames_to_file_infos.not_migrating_post.debug",
+    "translation": "Post already migrated to use FileInfos, post_id=%v, err=%v"
+  },
   {
     "id": "api.file.migrate_filenames_to_file_infos.save_file_info.warn",
-    "translation": "Unable to save post when migrating post to use FileInfos, post_id=%v, new_file_ids=%v, old_filenames=%v, err=%v"
+    "translation": "Unable to save post when migrating post to use FileInfos, post_id=%v, file_id=%v, path=%v, err=%v"
   },
   {
     "id": "api.file.migrate_filenames_to_file_infos.save_post.warn",