Fix race condition when deleting files

Gracefully handle multiple threads deleting an object at the same time.
When two concurrent requests come in to delete the last two remaining
objects in a given path, they'll be racing each other trying to delete
and cause a spurious deletion failure for one of them.

Fixes jamhall#788

lib/stores/filesystem.js

@@ -377,14 +377,13 @@ class FilesystemStore {
     const parts = key.split('/');
     // the last part isn't a directory (it's embedded into the file name)
     parts.pop();
-    while (
-      parts.length &&
-      !readdirSync(path.join(bucketPath, ...parts)).length
-    ) {
-      await fs.rmdir(path.join(bucketPath, ...parts));
-      parts.pop();
+    while (parts.length) {
+      await fs.rmdir(path.join(bucketPath, ...parts)).catch(err => {
+        if (err.code !== 'ENOENT' && err.code !== 'ENOTEMPTY') throw err;
+        parts.length = 0;
+      });
     }
-  }
+}
 
   async initiateUpload(bucket, key, uploadId, metadata) {
     const uploadDir = path.join(