Two races found by a second review round, both in the staging logic added by
the previous commit.
- Two in-flight sends of the same chunk index shared one `.part` path, so
whichever renamed first published bytes the other had already truncated. An
acknowledged 10-byte chunk could end up 2 bytes. The staging suffix is now
per-attempt rather than per-index.
- Unlinking the partial file raced the write stream's pending open(). destroy()
does not await it, so the unlink failed with ENOENT and the open then
recreated the `.part` file after cleanup had supposedly finished — reported
reproducible in 121 of 300 immediately-failing streams. Cleanup now waits for
the stream to close.
Relates to issue 1403
Paul Nothaft
b87ca5fba1