Large clones (e.g. nixpkgs) spend more than a minute selecting objects before sending any pack data. Proxies and clients might drop the connection for timeout. Like git's uploadpack.keepAlive, run selection on a worker thread and write an empty data-band packet every 5s until it finishes. Keepalives only go out on sideband connections, after the `packfile` line, so packfile-uris requests still select silently. Signed-off-by: Seongmin Lee <git@boltless.me>