feat: add the "maxPayload" field in the handshake details

So that clients in HTTP long-polling can decide how many packets they
have to send to stay under the maxHttpBufferSize value.

This is a backward compatible change which should not mandate a new
major revision of the protocol (we stay in v4), as we only add a field
in the JSON-encoded handshake data:

```
0{"sid":"lv_VI97HAXpY6yYWAAAC","upgrades":["websocket"],"pingInterval":25000,"pingTimeout":5000,"maxPayload":1000000}
```

Related: socketio/socket.io-client#1531

Author: darrachequesne

Diff for: lib/socket.ts

@@ -90,7 +90,8 @@ export class Socket extends EventEmitter {
         sid: this.id,
         upgrades: this.getAvailableUpgrades(),
         pingInterval: this.server.opts.pingInterval,
-        pingTimeout: this.server.opts.pingTimeout
+        pingTimeout: this.server.opts.pingTimeout,
+        maxPayload: this.server.opts.maxHttpBufferSize
       })
     );
 

Diff for: test/server.js

@@ -449,6 +449,7 @@ describe("server", () => {
           expect(obj.sid).to.be.a("string");
           expect(obj.pingTimeout).to.be.a("number");
           expect(obj.upgrades).to.be.an("array");
+          expect(obj.maxPayload).to.eql(1000000);
           done();
         });
       });
@@ -3356,7 +3357,7 @@ describe("server", () => {
             expect(res.headers["set-cookie"].length).to.be(2);
             expect(res.headers["set-cookie"][1]).to.be("mycookie=456");
 
-            const sid = JSON.parse(res.text.substring(4)).sid;
+            const sid = JSON.parse(res.text.substring(5)).sid;
 
             request
               .post(`http://localhost:${port}/engine.io/`)
@@ -3393,7 +3394,7 @@ describe("server", () => {
             expect(res.headers["set-cookie"].length).to.be(2);
             expect(res.headers["set-cookie"][1]).to.be("mycookie=456");
 
-            const sid = JSON.parse(res.text.substring(4)).sid;
+            const sid = JSON.parse(res.text.substring(5)).sid;
 
             request
               .post(`http://localhost:${port}/engine.io/`)