An accepted upload only ever dialled the downloader at the address the
server reported. Most downloaders sit behind a NAT, so the dial timed out
after 20s ("serve …: connection timed out") or, with port 0, the upload
sat in pending_serves forever. There was no fallback: an inbound
PierceFirewall was always treated as a P control connection.
Do what the reference clients do. When an upload starts, ask the server to
have the downloader connect back (ConnectToPeer type F) and dial them at
the same time; the first socket to arrive claims the transfer. Over the
pierced socket only the bare 4-byte transfer token precedes the offset
exchange. CantConnectToPeer and the brokered-connect timeout fail the
upload and free its slot; port 0 skips the dial instead of parking the job.
Covered by an e2e test where the leecher runs without a listener.
Closes#102
Michel de Graaf
3205611609