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/request.js

@@ -3,7 +3,7 @@ const assert = require('assert');
 
 const request = require('request');
 
-const {signMessage} = require('./utility.js');
+const {signMessage, getServerTimeOffset} = require('./utility.js');
 
 const baseUrls = {
   livenet: 'https://api.bybit.com',
@@ -14,9 +14,13 @@ module.exports = class Request {
 
   constructor(key, secret, livenet=false) {
     this.baseUrl = baseUrls[livenet === true ? 'livenet' : 'testnet'];
+    this._timeOffset = null;
 
     if(key) assert(secret, 'Secret is required for private enpoints');
 
+    this._syncTime();
+    setInterval(this._syncTime.bind(this), 3600000);
+
     this.key = key;
     this.secret = secret;
   }
@@ -34,12 +38,22 @@ module.exports = class Request {
     return result;
   }
 
+  async getTimeOffset() {
+    const start = Date.now();
+    const result = await this.get('/v2/public/time');
+    const end = Date.now();
+
+    return Math.ceil((result.time_now * 1000) - start + ((end - start) / 2));
+  }
+
   async _call(method, endpoint, params) {
     const publicEndpoint = endpoint.startsWith('/v2/public');
 
     if(!publicEndpoint) {
       if(!this.key || !this.secret) throw new Error('Private endpoints require api and private keys set');
 
+      if(this._timeOffset === null) await this._syncTime();
+
       params = this._signRequest(params);
     }
 
@@ -73,7 +87,7 @@ module.exports = class Request {
     const params = {
       ...data,
       api_key: this.key,
-      timestamp: Date.now()
+      timestamp: Date.now() + this._timeOffset
     };
 
     if(this.key && this.secret) {
@@ -89,4 +103,8 @@ module.exports = class Request {
       .map(key => `${key}=${params[key]}`)
       .join('&');
   }
+
+  async _syncTime() {
+    this._timeOffset = await this.getTimeOffset();
+  }
 }