use Date.now() instead of new Date().getTime()

This is documented to be more performant.

The substitution was made on frontend code, too (i.e., the one in /static),
because Date.now() is supported since IE 9, and we are life supporting only
IE 11.

Commands:
  find . -name *.js | xargs sed --in-place "s/new Date().getTime()/Date.now()/g"
  find . -name *.js | xargs sed --in-place "s/(new Date()).getTime()/Date.now()/g"

Not done on jQuery.

src/node/db/API.js

@@ -522,7 +522,7 @@ exports.appendChatMessage = function(padID, text, authorID, time, callback)
   if(time === undefined || !is_int(time))
   {
     // set time to current timestamp
-    time = new Date().getTime();
+    time = Date.now();
   }
 
   var padMessage = require("ep_etherpad-lite/node/handler/PadMessageHandler.js");