Socket.nsp to public readonly

Manager will remove the namespace'd socket upon socket disconnect, thus allowing subsequent connections to the same namespace to multiplex.

lib/manager.ts

@@ -476,7 +476,8 @@ export class Manager<
    * @param socket
    * @private
    */
-  _destroy(socket: Socket): void {
+  _destroy(closingSocket: Socket): void {
+    delete this.nsps[closingSocket.nsp];
     const nsps = Object.keys(this.nsps);
 
     for (const nsp of nsps) {