replace ServerRegistration#unfurlSrv to DNSResolver class

lib/time.js

@@ -7,7 +7,7 @@ class TimeTracker {
   }
 
   newTimestamp () {
-    const timestamp = new Date().getTime()
+    const timestamp = TimeTracker.getEpochMillis()
 
     this._points.push(timestamp)
 
@@ -22,6 +22,10 @@ class TimeTracker {
     return this._points
   }
 
+  static getEpochMillis () {
+    return new Date().getTime()
+  }
+
   static getMaxServerGraphDataLength () {
     return Math.ceil(config.serverGraphDuration / config.rates.pingAll)
   }