Currently, each request to purge cache by tags triggers a request to "/_ez_http_invalidatetoken" route
A response of this route should be cached by varnish, but it's not.
It's caused by session usage, and the fact this response does not vary by user context.

Steps to reproduce
1) trigger cache clear first time

php bin/console fos:httpcache:invalidate:tag c638364

2) check nginx access logs and observe request to "/_ez_http_invalidatetoken"
3) trigger cache clear again

php bin/console fos:httpcache:invalidate:tag c638364

AR:
4) check nginx access logs and observe request to "/_ez_http_invalidatetoken"

ER:
4) check nginx access logs and don't see a new request to "/_ez_http_invalidatetoken", as varnish cache reused.

PR: coming soon