socket: always pass socket, fd and SocketPeer ownership to service together

socket: always pass socket, fd and SocketPeer ownership to service together

Per-connection socket instances we currently maintain three fields
related to the socket: a reference to the Socket unit, the connection fd,
and a reference to the SocketPeer object that counts socket peers.

Let's synchronize their lifetime, i.e. always set them all three
together or unset them together, so that their reference counters stay
synchronous.

THis will in particuar ensure that we'll drop the SocketPeer reference
whenever we leave an active state of the service unit, i.e. at the same
time we close the fd for it.

Fixes: #20685