This is a back port of 33143e05 in
master, and for this reason it is a little strange.

The strangeness is due to the fact that obj.ttl is not available
in vcl_deliver here, but it is in master. This commit could have
been much simpler without ttl_now, but the function is taken to
4.1 regardles. The reason is that introducing obj.ttl in
vcl_deliver is straightforward, and if someone is to do that in
the future, the code in ttl_now(VRT_CTX) makes sure that obj.ttl
will behave as in master, also in vcl_deliver.

If obj.ttl is introduced in vcl_deliver, than also the two test
cases s00008.vtc and s00009.vtc should be brought in, to make sure
that obj.ttl works as expected.

The following is the test from the commit in master:

A new fucntion, ttl_now(VRT_CTX), defines what "now" is when ttl
and age are calculated in various VCL subs. To sum up,

* Before a backend fetch on the client side (vcl_recv, vcl_hit,
  vcl_miss) we use t_req from the request. This is the significance
  in this commit, and fixes the bug demonstrated by r02555.vtc.
* On the backend side, most notably vcl_backend_responce, we keep
  the old "now" by simply using ctx->now.
* In vcl_deliver we use ctx->now, as before.

It was necessary to make all purges use t_req as their base time.
Then, to not break c00041.vtc it was necessary to change from ">="
to ">" in HSH_Lookup.

All VMODs that currently use HSH_purge must change to using
VRT_purge.

Conflicts:
	bin/varnishd/cache/cache_hash.c
	bin/varnishd/cache/cache_objhead.h
	bin/varnishd/cache/cache_req_fsm.c
	bin/varnishd/cache/cache_vrt.c
	bin/varnishd/cache/cache_vrt_var.c