feat(lib): let another thread cancel a client's connect and login

A caller that gives up on a login had no way to reach the client: the
thread that owns it sits in login() for up to 45 seconds, and disconnect()
needs the client to itself. Until that thread returned, the abandoned
client kept connecting and still wrote its login frame the moment the
server finished the TCP handshake. When that happened after a replacement
had logged in under the same name, the server kicked the replacement.

Client::cancel_handle() hands out a CancelHandle that can be moved to any
thread. After cancel():
- the server actor writes nothing more. The check and the write share a
  lock with cancel(), so once cancel() returns no frame leaves, whether it
  was waiting in the queue, in the dispatcher channel, or mid-flush;
- login() returns NotConnected within 100 ms, and hands no login to the
  actor at all when the client was already cancelled;
- connect() refuses before binding the listener when the client was
  already cancelled. A cancel that lands during that check can still let
  the bind through.
Either way the owner drops the client as before, which disconnects it and
gives its port back.

A frame already written before cancel() returned cannot be recalled. The
API is additive, and login() on a client nobody cancels behaves as before.

Soulseek.NET tears the whole connection down when ConnectAsync is
cancelled, and Nicotine+ drops outgoing messages for a socket it closed;
this gives soulseek-rs the same end state from another thread.

soulseek-rs-lib/src/client/mod.rs

@@ -649,6 +649,7 @@ pub struct Client {
     /// port it bound.
     listener_stopped: Arc<AtomicBool>,
     listener_released: Arc<AtomicBool>,
+    cancel: CancelHandle,
 }
 
 impl Drop for Client {
@@ -684,6 +685,7 @@ impl Client {
             session: SessionWatch::default(),
             listener_stopped: Arc::new(AtomicBool::new(false)),
             listener_released: Arc::new(AtomicBool::new(false)),
+            cancel: CancelHandle::new(),
         }
     }
 
@@ -724,6 +726,11 @@ impl Client {
         self.session.loss()
     }
 
+    #[must_use]
+    pub fn cancel_handle(&self) -> CancelHandle {
+        self.cancel.clone()
+    }
+
     /// The directories whose files are currently shared with other peers.
     #[must_use]
     pub fn shared_directories(&self) -> Vec<String> {
@@ -849,6 +856,7 @@ impl Client {
     }
 }
 
+mod cancel;
 mod children;
 mod connection;
 mod context;
@@ -861,5 +869,7 @@ mod social;
 mod upload_queue;
 mod uploads;
 
+pub use cancel::CancelHandle;
+
 #[cfg(test)]
 mod tests;