feat(interests): keep our own list and send it again each login

The server holds interests only for the session that set them, so a client
that does not re-send its list comes back from a reconnect with none. That
is why Nicotine+ sends its whole list at every login, and why it lowercases
each item before sending — the server matches them case-insensitively, so
two spellings would otherwise be held as two interests.

Ours now keeps the list, in one spelling, and puts it back on the wire when
a session starts; an interest set before there is a connection is kept
rather than dropped. A room's ticker board is also let go when we leave the
room, since the server sends the whole board again on the next join.

soulseek-rs-lib/src/actor/server_actor.rs

@@ -834,6 +834,8 @@ impl ServerActor {
             // The distributed stance is the leaf's to announce, and a new
             // session starts without a parent.
             self.forward_to_client(ClientOperation::ResetDistributed);
+            // The server holds interests only for the life of a session.
+            self.forward_to_client(ClientOperation::SessionEstablished);
         }
         match self.context.write_safe() {
             Ok(mut ctx) => ctx.logged_in = Some(message),

soulseek-rs-lib/src/actor/server_actor/tests.rs

@@ -210,6 +210,12 @@ fn a_successful_login_resets_the_distributed_leaf() {
         client.try_recv(),
         Ok(ClientOperation::ResetDistributed)
     ));
+    // A new session also has to be told what only the session held: our
+    // interests, which the server forgets when the old one ended.
+    assert!(matches!(
+        client.try_recv(),
+        Ok(ClientOperation::SessionEstablished)
+    ));
     actor.handle_login_status(false);
     assert!(client.try_recv().is_err(), "a failed login resets nothing");
 }