Verify CSRF token before accepting new CSRF connection (#11635)

* Can't open push connection during client-side init because CSRF token
is not available at that point. This allows simplifying the
initialization because the push state will not be checked until the
first response has been processed.
* Add helper for checking the CSRF token

Change-Id: I31da1ac669dc9a581cbd66f58c07f10ea4b8b676