calling piwikTracker.getVisitorId() early causes random ID to return, not persisted #2315

when calling JS API piwikTracker.getVisitorId() early (before the trackPageView() call) the returned ID will be a random one, not actually used by Piwik later. Instead, loadVisitorCookie should set the cookie when it's not already set.

(In [4462]) Fixes #2315 and updating tests (starting to love these JS tests ;)

The code should only persist uuid (in a variable).

The current code fails for a new visitor when you call getVisitorId() and then trackPageView() because the saved cookie will incorrectly be treated as a return visitor.

(In [4475]) fixes #2315