VCL: v4 deliver+synth refactoring
This ensures certain cluster-shared vcl_deliver functionality for
important functional/debug/analytics headers (X-Carrier,
X-Client-IP, X-Analytics, X-Cache, X-Connection-Properties, Via,
X-Varnish, HSTS) is also invoked in vcl_synth under varnish4, as
some classes of request flow skip vcl_deliver in varnish4 in favor
of vcl_synth, but these are all still applicable in that case.