Possible Timing Side-Channel in XMLRPC Verification

https://github.com/Automattic/jetpack/blob/bc7a4541ef6f0e9f583376d801ab0c40cfb976c3/class.jetpack-xmlrpc-server.php#L115 I mentioned this to @daljo628 and he suggested submitting it here instead. This looks very much like a classic [timing attack vulnerability](http://blog.ircmaxell.com/2014/11/its-all-about-time.html). The fix would be to use `hash_equals()` (which I have provided a sane polyfill for in [sarciszewski/php-future](https://github.com/sarciszewski/php-future) if you don't already have one handy).