All upstream GET/HEAD and bearer-token requests used http.DefaultClient,
which has no timeouts, so a slow or wedged upstream could pin a goroutine
and connection indefinitely. Introduce a shared upstreamClient with
dial, TLS-handshake and response-header timeouts (no overall Client
timeout, so large artifact bodies can still stream, bounded by the
request context).
Refs #67
unkinben
1476120c7b