Avoid conccurent _syncTime calls

lib/request.js

@@ -15,6 +15,7 @@ module.exports = class Request {
   constructor(key, secret, livenet=false) {
     this.baseUrl = baseUrls[livenet === true ? 'livenet' : 'testnet'];
     this._timeOffset = null;
+    this._syncTimePromise = null;
 
     if(key) assert(secret, 'Secret is required for private enpoints');
 
@@ -105,6 +106,18 @@ module.exports = class Request {
   }
 
   async _syncTime() {
-    this._timeOffset = await this.getTimeOffset();
+    if(this._syncTimePromise !== null) return this._syncTimePromise;
+
+    this._syncTimePromise = new Promise(async (resolve, reject) => {
+      try {
+        this._timeOffset = await this.getTimeOffset();
+        this._syncTimePromise = null;
+        resolve();
+      } catch(err) {
+        reject(err);
+      }
+    });
+
+    return this._syncTimePromise;
   }
 }