Exposing flushcookie api.

Been spending lots of time with the cookie API and notice we don't have this API exposed . 

Put together ```b5319b0``` for it.

I modeled this after the VisitAllCookies API design.

More more more cookies :cookie: :cookie: :cookie: 

Also noticed we are not setting the creation time of the cookies in the visit API , pulling that information out of the struct and adding to the .net cookies in ```63f13a4```.

Cookie time :cookie: