fix: avoid races in NewMultipartUpload under multiple pools (#12233)

It is possible in some scenarios that in multiple pools,
two concurrent calls for the same object as a multipart operation
can lead to duplicate entries on two different pools.

This PR fixes this

- hold locks to serialize multiple callers so that we don't race.
- make sure to look for existing objects on the namespace as well
  not just for existing uploadIDs

cmd/api-errors.go

@@ -1082,9 +1082,9 @@ var errorCodes = errorCodeMap{
 		HTTPStatusCode: http.StatusBadRequest,
 	},
 	ErrKMSNotConfigured: {
-		Code:           "InvalidArgument",
+		Code:           "NotImplemented",
 		Description:    "Server side encryption specified but KMS is not configured",
-		HTTPStatusCode: http.StatusBadRequest,
+		HTTPStatusCode: http.StatusNotImplemented,
 	},
 	ErrNoAccessKey: {
 		Code:           "AccessDenied",