Time synchronization with server

- wrapper for /v2/public/time rest endpoint
- synchronize time before trying to authenticate a request in order
  to avoid invalid authentication because prevented replay attacks.

Resolves #1

lib/websocket-client.js

@@ -3,6 +3,7 @@ const {EventEmitter} = require('events');
 const WebSocket = require('ws');
 
 const defaultLogger = require('./logger.js');
+const RestClient = require('./rest-client.js');
 const {signMessage} = require('./utility.js');
 
 const wsUrls = {
@@ -33,6 +34,8 @@ module.exports = class WebsocketClient extends EventEmitter {
       ...options
     }
 
+    this.client = new RestClient(null, null, this.options.livenet);
+
     this._connect();
   }
 
@@ -54,10 +57,11 @@ module.exports = class WebsocketClient extends EventEmitter {
     this.ws.close();
   }
 
-  _connect() {
+  async _connect() {
     if(this.readyState === READY_STATE_INITIAL) this.readyState = READY_STATE_CONNECTING;
 
-    const url = wsUrls[this.options.livenet ? 'livenet' : 'testnet'] + this._authenticate();
+    const authParams = await this._authenticate();
+    const url = wsUrls[this.options.livenet ? 'livenet' : 'testnet'] + authParams;
 
     this.ws = new WebSocket(url);
 
@@ -67,12 +71,15 @@ module.exports = class WebsocketClient extends EventEmitter {
     this.ws.on('close', this._wsCloseHandler.bind(this));
   }
 
-  _authenticate() {
+  async _authenticate() {
     if(this.options.key && this.options.secret) {
       this.logger.debug('Starting authenticated websocket client.', {category: 'bybit-ws'});
+
+      const timeOffset = await this.client.getTimeOffset();
+
       const params = {
         api_key: this.options.key,
-        expires: (Date.now() + 10000)
+        expires: (Date.now() + timeOffset + 5000)
       };
 
       params.signature = signMessage('GET/realtime' + params.expires, this.options.secret);