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/SecurityManager.js

@@ -152,7 +152,7 @@ exports.checkAccess = function (padID, sessionCookie, token, password, callback)
               
               if(ERR(err, callback)) return;
               
-              var now = Math.floor(new Date().getTime()/1000);
+              var now = Math.floor(Date.now()/1000);
               
               //is it for this group?
               if(sessionInfo.groupID != groupID)