This is a back port of ff38535a

The req.grace variable can be set in vcl_recv to cap the grace
of objects in the cache, in the same way as in 3.0.x

The "keep" behavior changes with this patch. We now always go
to vcl_miss when the expired object is out of grace, or we go
to the waiting list. The result is that it is no longer
possible to deliver a "keep" object in vcl_hit.

Note that when we get to vcl_miss, we will still have the 304
candidate, but without the detour by vcl_hit.

This commit changes VCL, but only slightly, so we aim to back
port this to earlier versions of Varnish Cache.

Refs: #1799 and #2519

Conflicts:
	bin/varnishd/cache/cache_hash.c
	bin/varnishd/cache/cache_req_fsm.c
	bin/varnishd/cache/cache_varnishd.h
	bin/varnishd/cache/cache_vrt_var.c
	doc/sphinx/reference/vcl_var.rst