feat: increase the default value of pingTimeout

This value was updated from 60000 to 5000 in [1], included in
`engine.io@3.2.0` (Feb 2018).

The reasoning back then:

Some users experienced long delays between disconnection on the
server-side and on the client-side. The "disconnect" event would take a
long time to fire in the browser, probably due to a timer being
delayed. Hence the change.

That being said, the current value (5s) now causes unexpected
disconnections when a big payload is sent over a slow network, because
it prevents the ping-pong packets from being exchanged between the
client and the server. This can also happen when a synchronous task
blocks the server for more than 5 seconds.

The new value (20s) thus seems like a good balance between quick
disconnection detection and tolerance to various delays.

Note: pingInterval + pingTimeout is still below the threshold of React
Native, which complains if a timer is set with a delay of more than 1
minute.

[1]: 65b1ad1

Related:

- socketio/socket.io#2770
- socketio/socket.io#2769
- socketio/socket.io#3054
- socketio/socket.io#3376

Backported from 5a7fa13

README.md

@@ -221,7 +221,7 @@ to a single process.
       - `Object`: optional, options object
     - **Options**
       - `pingTimeout` (`Number`): how many ms without a pong packet to
-        consider the connection closed (`5000`)
+        consider the connection closed (`20000`)
       - `pingInterval` (`Number`): how many ms before sending a new ping
         packet (`25000`)
       - `upgradeTimeout` (`Number`): how many ms before an uncompleted transport upgrade is cancelled (`10000`)

lib/server.js

@@ -37,7 +37,7 @@ function Server (opts) {
   opts = opts || {};
 
   this.wsEngine = opts.wsEngine || process.env.EIO_WS_ENGINE || 'ws';
-  this.pingTimeout = opts.pingTimeout || 5000;
+  this.pingTimeout = opts.pingTimeout || 20000;
   this.pingInterval = opts.pingInterval || 25000;
   this.upgradeTimeout = opts.upgradeTimeout || 10000;
   this.maxHttpBufferSize = opts.maxHttpBufferSize || 1e6;