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/tests/resume_e2e.rs

@@ -67,6 +67,7 @@ impl Subject {
             enable_listen: true,
             listen_port: 0,
             shared_directories: Vec::new(),
+            accept_children: false,
             version: ClientVersion::default(),
         });
         client.connect().expect("connect subject");