VCC's own complaint instead.

The latter takes precedence as per RFC 7232.  Also if we have an I-N-M
but no entity don't check I-M-S, just do a non-conditional request.

That is, if both If-None-Match and Range are present we need to use a
strong comparison as the representation needs to be byte-for-byte
identical.

As per RFC 7232.

Fixes #1816.

Send your kudos to POSIX for failing to provide a standard here.

with a single one.

It's needed for the vdi_healthy_f invocation.

This is for the 4.1 ABI, since the next major release will have a
VRT_MAJOR_VERSION bump instead. The two functions may also change
semantics before the next 4.1.x, but will still be _new_ as far as
releases are concerned.

By default the reference is released as soon as the VCL goes cold, which
doesn't harm the existing tests yet allows to cover the new VRT_ref_vcl
and VRT_rel_vcl functions.

It is possible to ask the vmod to release the reference after a delay.

Now that VCLs can be referenced, there are two different situations
stalling the cooldown. The "cooling" one is kept for VCLs that have
received a COLD event, and the "busy" one is introduced to wait for
ongoing transactions.

Basically, if there are transactions, the VCL is still active even
though it's not *the* active VCL and should therefore be considered
warm. This guarantees that WARM and COLD events are balanced, and
also that a "busy" VCL doesn't need to warm up if it is set to warm
again.

The new VCL temperature engine looks like this:

  .----------W>---------------.
  |              .--W>---.    v
init ---> cold --|       |-- warm --.
          ^  ^   '---<C--'    |     |
          |  |                |     |
          |  '--- cooling <C--'     |
          |         ^               |
          |         C               |
          |         |    .---<---.  |
          '--<C-- busy --|       |--'
                         '--->---'

The transitions marked with a 'W' or a 'C' are the one dispatching
WARM and COLD events respectively.

Since the "busy" state is considered warm, VMODs should operate as
such and backend creation remains possible.

Broken implementations do it contrary to what the RFC mandates.

Fixes #1826.