This fixes the long-standing #1799 for "keep" objects, and this commit
message suggests a way of working around #1799 in the remaining
cases. The following is a (long) explanation on how grace and keep
works at the moment, how this relates to #1799, and how this commit
changes things.

1. How does it work now, before this commit?

Objects in cache can outlive their TTL, and the typical reason for
this is grace. Objects in cache can also linger because of obj.keep or
in the (rare but observed) case where the expiry thread have not yet
evicted an object. Grace and keep are here to minimize backend load,
but #1799 shows that we are not successful in doing this in some
important cases.

Whenever sub vcl_recv has ended with return (lookup) (which is the
default action), we arrive at HSH_Lookup, where varnish sometimes only
finds an expired object (that match Vary logic, is not banned,
etc). When this happens, we will initiate a background fetch (by
adding a "busy object") if and only if there is no busy object on the
oh already. Then the expired object is returned with HSH_EXP or
HSH_EXPBUSY, depending on whether a busy object was inserted.

2. What makes us run into #1799?

When we have gotten an expired object, we generally hope that it is in
grace, and that sub vcl_hit will return(deliver). However, if grace
has expired, then the default action (ie the action from builtin.vcl)
is return (miss). It is also possible that the user vcl, for some
reason, decides that the stale object should not be delivered, and
does return (miss) explicitly. In these cases it is common that the
current request is not the one to insert a busy object, and then we
run into the issue with a message "vcl_hit{} returns miss without busy
object. Doing pass.".

Note that normally, if a resource is very popular and has a positive
grace, it is unlikely that #1799 will happen. Then a new version will
always be available before the grace has run out, and everybody get
the latest fetched version with no #1799 problems.

However, if a resource is very popular (like a manifest file in a live
streaming setup) and has 0s grace, and the expiry thread lags a little
bit behind, then vcl_hit can get an expired object even when obj.keep
is zero. In these circumstances we can get a surge of requests to the
backend, and this is especially bad on a very busy server.

Another real world example is where grace is initially set high (48h
or similar) and vcl_hit considers the health of the backend, and, if
the backend is healthy, explicitly does a return(miss) ensure that the
client gets a fresh object. This has been a recommended use of
vcl_hit, but, because of #1799, can cause considerable load on the
backend.

Similarly, we can get #1799 if we use "keep" to facilitate IMS
requests to the backend, and we have a stale object for which several
requests arrive before the first completes.

3. How do we fix this?

The main idea is to teach varnish to consider grace during lookup.

To be specific, the following changes with this commit: If an expired
object is found, the ttl+grace has expired and there already is an
ongoing request for the object (ie. there exists a busy object), then
the request is put on the waiting list instead of simply returning the
object ("without a busy object") to vcl_hit. This choice is made
because we anticipate that vcl_hit will do (the default) return (miss)
and that it is better to wait for the ongoing request than to initiate
a new one with "pass" behavior.

The result is that when the ongoing request finishes, we will either
be able to go to vcl_hit, start a new request (can happen if there was
a Vary mismatch) by inserting a new "busy object", or we lose the race
and have to go back to the waiting list (typically unlikely).

When grace is in effect we go to vcl_hit even when we did not insert a
busy object, anticipating that vcl_hit will return (deliver).

This will will fix the cases where the user does not explicitly do a
return(miss) in vcl_hit for object where ttl+grace has not
expired. However, since this is not an uncommon practice, we also have
to change our recommendation on how to use grace and keep. The new
recommendation will be:

* Set grace to the "normal value" for a working varnish+backend.

* Set keep to a high value if the backend is not 100% reliable and you
  want to use stale objects as a fallback.

* Do not explicitly return(miss) in sub vcl_hit{}. The exception is
  when this only can happen now and then and you are really sure that
  this is the right thing to do.

* In vcl_hit, check if the backend is sick, and then explicitly
  return(deliver) when appropriate (ie you want an stale object
  delivered instead of an error message).

A test case is included.