the be->vsc under backend mtx.

Fixes #2505

Also add asserts for the references held in req->objcore and
req->stale_oc.

The test case for #1807 catches this bug after adding the asserts.

Fixes: #2502

Fixes: #1772

With VTP_Close now being capable of dealing with STOLEN connections, we
no longer need to VTP_Wait for them prior to close.

The change from shutdown(.., SHUT_WR) to shutdown(.., SHUT_RDWR) is
required to make it trigger a waiter event.

This test case should not rely on
first_byte_timeout/between_bytes_timeout.

This issue is hidden by the fix in the previous commits, so we cannot test
for it directly any more. Applying this patch to
fa388439 exposes the "Constructor not found".

Fixes #2498

When we are looking for ! SYM_NONE and find ! SYM_NONE, do not
return the symbol we found.

The semantics of (kind != SYM_NONE && sym->kind != kind) are
inclusive because the next conditional is not true for this case.

Ref: 9d81a71a

Fixes: #2497

The second time around, we force a fresh connection.

The VCL user may choose to do 'return (retry);' in vcl_backend_error{}
if further attempts are deemed warranted.

Fixes: #2135

Passing a nonzero value will force a fresh backend connection.

that we now actually *do* get the "child panicked" exit code.

it uses a 'double' which only has approx 52 bits of mantissa, which is
marginal already for PetaBytes.

Test various edge-cases.

The counter cache_hit_grace counts the number of grace hits. To be
precise, it counts the number of times lookup returns an expired
object, but vcl_hit is called and decides to return(deliver).

Every time cache_hit_grace is incremented, cache_hit is also
incremented (so this commit does not change the cache_hit counter).

I'm guessing this is due to rounding. All test cases involving file
stevedore has a minimum 10m file in the test, was silly to attempt a
smaller one in this test.

Fixes: #2496

of sourcefile names over the project.

Fixes #2448

for example, mgt_shm_atexit could race in two processes, one in
VSMW_Destroy and the other in system("rm -rf")

Closes #2484

... now that I've found the layer 8 bug

Thank you to @bsdphk

Ref #2484

I had left in superfluous stuff from bug analysis.

Ref: #2319

Creating "backend synthetic" content was impossible whenever a fetch
had already started, unless storage was assigned (again) explicitly
from VCL.

Fixes #2494