Increase default listen backlog from 5 to 128
The accept queue was limited to 5 pending connections, which overflows
easily under connection churn or a burst of simultaneous connects (LB
health checks, thundering herd on restart). On overflow the kernel
silently drops the ACK rather than failing fast, so clients stall on
SYN/ACK retransmission backoff.
Benchmark (bombardier -c 10 -d 10s, 3 trials, interleaved with the old
binary) shows max latency dropping from 48-89ms to 5.8-11.2ms, while
p99 is unchanged - the fix affects only the extreme tail, as expected.
yhirose