Allow persistent connections for HTTP PURGE (error) responses

Varnish always closes the HTTP connection when going through vcl_error.
Because we use vcl_error for things like purging, geoip and redirects,
this is suboptimal. We've patched Varnish to support persistent
connections when the object's Connection: response header is set to
"keep-alive", and the client allows it.

Varnish overwrites the Connection: response header if it
determines it needs to close anyway, so no need to be careful in
VCL for it.

Change-Id: I33ea1753ce37512f7532e090e426a29f351bcd3e