Lock bucket while modifying its metadata (Fixes #2979) (#3019)

- When modifying notification configuration
- When modifying listener configuration
- When modifying policy configuration

With this change we also stop early checking if the bucket exists, since
that uses a Read-lock and causes a deadlock due to the outer Write-lock.

cmd/event-notifier.go

@@ -399,12 +399,6 @@ func persistNotificationConfig(bucket string, ncfg *notificationConfig, obj Obje
 		return err
 	}
 
-	// verify bucket exists
-	// FIXME: There is a race between this check and PutObject
-	if err = isBucketExist(bucket, obj); err != nil {
-		return err
-	}
-
 	// build path
 	ncPath := path.Join(bucketConfigPrefix, bucket, bucketNotificationConfig)
 	// write object to path
@@ -425,12 +419,6 @@ func persistListenerConfig(bucket string, lcfg []listenerConfig, obj ObjectLayer
 		return err
 	}
 
-	// verify bucket exists
-	// FIXME: There is a race between this check and PutObject
-	if err = isBucketExist(bucket, obj); err != nil {
-		return err
-	}
-
 	// build path
 	lcPath := path.Join(bucketConfigPrefix, bucket, bucketListenerConfig)
 	// write object to path