fix(electrum): handle shutdown race in acceptor thread

  The 'acceptor' thread panicked with 'send failed' when a TCP connection
  arrived at the exact moment the server was shutting down. The 'rpc'
  thread drops the channel receiver upon receiving the Exit notification,
  leaving the 'acceptor' thread's next send() with no receiver.

  - Break cleanly from the acceptor loop when the receiver is dropped
  - Replace unwrap() with a warn! log in the notification thread for the
    symmetric case where the acceptor has already exited before Exit fires

Author: EddieHouston

src/electrum/server.rs

@@ -766,7 +766,11 @@ impl RPC {
                             }
                         })
                     }
-                    Notification::Exit => acceptor.send(None).unwrap(), // mark acceptor as done
+                    Notification::Exit => {
+                        if acceptor.send(None).is_err() {
+                            warn!("acceptor already shut down before Exit notification");
+                        }
+                    }
                 }
             }
         });
@@ -789,7 +793,9 @@ impl RPC {
                 stream
                     .set_nonblocking(false)
                     .expect("failed to set connection as blocking");
-                acceptor.send(Some((stream, addr))).expect("send failed");
+                if acceptor.send(Some((stream, addr))).is_err() {
+                    break; // receiver dropped, server is shutting down
+                }
             }
         });
         chan