[CB-6389] add clearsessioncache option to iOS (like Android)

I'd like to add the clearsessioncache option to iOS that Android has.  The feature appears to be rather uncomplicated.  If clearsessioncache is turned on, then when a url is open in the appbrowser, before it loads, I clear out all the cookies.  This should effectively clear out any session that existed before, allowing for a clean slate.