feat(distributed): serve children in the search tree

The client read the distributed network but never carried it: a peer
dialling with a D connection was logged and dropped, and AcceptChildren
was hardcoded false, so this client took from the tree without ever
relaying a search for anyone else.

With accept_children set, peers hang from us: each is told where our
branch sits when it connects and again when the branch moves, every
search that reaches us is passed down, and a child that goes away loses
its slot. Ten at a time, matching Nicotine+, since each costs a socket
and a copy of the network's whole search stream — which is also why it is
opt-in, through the new accept_children config key (ignored without a
listener, since a child would have nothing to dial).

soulseek-rs-lib/examples/stress.rs

@@ -1026,6 +1026,7 @@ fn main() {
         enable_listen: true,
         listen_port: client_port,
         shared_directories: vec![share_dir.display().to_string()],
+        accept_children: false,
         version: ClientVersion::default(),
     });
     client.connect().expect("client connect");