We only need to wake up vbp_thread if the probe we insert on the binheap
has become the next due. As a binheap_root() call is cheap and saves
posting many signals, this optimization should be worth some extra
lines.

Also we add a function for the two places where we insert into the heap.

Ref #2976

We could also move the VSL() outside if we copied dir->vcl_name

The due time of the probe which has just finished may be earlier than
the previously earliest due time, so vbp_thread needs to be woken up to
update the nxt value

Fixes #2976

Via lgtm.

This reverts commit e7d5fb68.

Apparently this change looks intentional, but it should be revisited as
it makes troubleshooting significantly harder when logs (for example
Backend_health records) aren't explicit about the backend they are
referring to.

Instead of looking like the backend's VCL name we only get the symbol
name in logs, unlike the CLI. This brings back the full ${vcl}.${symbol}
name everywhere.

the per-release documentation and rearrange things a bit.

Competent review is requested.

This one is much more tricky to use though, certainly not for
automation purposes.

Slightly enhanced compared to when they were first used. I ran them and
found more opportunities to polish the code base.

I also decided to lowercase semantic patch file names, and to not
mislead people about the WS_Reserve change being a vmod-only thing.

Add Project Governance info.

Closes #2971

We handle the error case just above

Ref:
https://github.com/varnishcache/varnish-cache/pull/2969#pullrequestreview-224802203

Fixes #2946 in the sense that we want to treat a cacheable backend synth
like any other object and kill the stale object it replaces.

We do not replace the stale oc when
- abandoning the bereq
- leaving vcl_backend_error with return (deliver) and beresp.ttl == 0s
- there is a waitinglist on this object because in this case the default
  ttl would be 1s, so we might be looking at the same case as the
  previous

Thank you @mbgrydeland for the review and spotting a mistake.

Historically, all initializers called from VGC_Load() would handle
failures individually (return (1) from VGC_Load) or not fail at
all. Now that we got VRT_fail(), it makes sense to generalize.

The upcoming use case is to properly handle failures from
VRT_new_backend_clustered().

WS_ReserveSize() does not leave the workspace reserved when the
reservation fails, so WS_Release() must be called for retval > 0 only.

Besides the debug string, it is identical to WS_Reserve() except for

	assert(bytes > 0);

Follow-up #2967

This works with gcc 6.3.0 and clang 3.8.1-24

The test for __GNUC__ is deliberately simple and might not catch all
compilers which would potentially support deprecation marks. While more
specifics could be added, the aim is to raise awareness with developers
and we consider it quite unlikely that anyone does not compile with one
of the main stream compilers at all.

Seen fail on travis:

**   c1    2.7 === expect resp.status == 200
**** c1    2.7 EXPECT resp.status (200) == "200" match
**   c1    2.7 === expect resp.http.Age == 1
---- c1    2.7 EXPECT resp.http.Age (2) == "1" failed

To keep track of the number of times pipe_sess_max kicked in. This one
can be lazily updated.

Optionally, it can limit the number of concurrent pipe transactions as
they each monopolize one worker thread.

This allows users to keep track of the number of ongoing pipe
transactions at any time.