put_object_part's commit phase held an exclusive write lock on the whole
upload_id_path namespace, so concurrent UploadPart commits for different
part numbers of one upload serialized behind a single lock and returned
503 once the 5s lock-acquire timeout elapsed.
Adopt MinIO's PutObjectPart lock scope: a shared read lock on the
uploadId namespace plus an exclusive write lock on
{upload_id_path}/part.{N}. Different part numbers now commit
concurrently; same-part retries still serialize (backlog#853);
complete/abort keep the uploadId write lock and still exclude every
in-flight part commit. The lock-loss fence covers both guards.
Fixes#5961
Zhengchao An
1021d7228a