fix: only set 'connected' to true after middleware execution

The Socket instance is only considered connected when the "connection"
event is emitted, and not during the middleware(s) execution.

```js
io.use((socket, next) => {
  console.log(socket.connected); // prints "false"
  next();
});

io.on("connection", (socket) => {
  console.log(socket.connected); // prints "true"
});
```

Related: #4129

Backported from 02b0f73

lib/socket.js

@@ -66,8 +66,8 @@ function Socket(nsp, client, query){
   this.conn = client.conn;
   this.rooms = {};
   this.acks = {};
-  this.connected = true;
-  this.disconnected = false;
+  this.connected = false;
+  this.disconnected = true;
   this.handshake = this.buildHandshake(query);
   this.fns = [];
   this.flags = {};
@@ -300,6 +300,8 @@ Socket.prototype.leaveAll = function(){
 
 Socket.prototype.onconnect = function(){
   debug('socket connected - writing packet');
+  this.connected = true;
+  this.disconnected = false;
   this.nsp.connected[this.id] = this;
   this.join(this.id);
   var skip = this.nsp.name === '/' && this.nsp.fns.length === 0;

test/socket.io.js

@@ -2442,6 +2442,25 @@ describe('socket.io', function(){
         if (++count === 2) done();
       });
     });
+
+    it("should only set `connected` to true after the middleware execution", (done) => {
+      const httpServer = http();
+      const sio = io(httpServer);
+
+      const clientSocket = client(httpServer, "/");
+
+      sio.use((socket, next) => {
+        expect(socket.connected).to.be(false);
+        expect(socket.disconnected).to.be(true);
+        next();
+      });
+
+      sio.on("connection", (socket) => {
+        expect(socket.connected).to.be(true);
+        expect(socket.disconnected).to.be(false);
+        done();
+      });
+    });
   });
 
   describe('socket middleware', function(done){