(Same as we do in H1)

Fixes #2589

Spotted by:	Willy Tarreau <w@1wt.eu>

Fixes #2787

my apologies: as long as we pass around a struct wrk, some other
function could use the wrk->stats - in other words, the fact that
the ban stats are decoupled from the wrk stats does not make the
latter any less relevant.

This reverts commit 527f1bd0.

Checking and preparing our worker struct does not need to happen
under the lock.

Over time, I have repeatedly stared at this code again and again
wondering if (and why) our cv signaling is correct, just to end up
with the same insight each time (but first overlooking #2719)

Being fully aware that we do not want to plaster our code with
outdated comments, I hope this explanation is warranted to save myself
(and others, hopefully) from wasting precious life time on reiterating
over the same question.

Fixes: #2782

Take 2. Let's see if this time sticks.

Restructured so that:

* 'Upgrading' is limited to work that has to be done to upgrade from
  a current deployment to the new version.

* 'Changes' is a comprehensive, user-level description of changes and
  new features.

This adds -Wno-implicit-fallthrough for libvgz to satisfy gcc7.

The -Wno-unknown-warning-option is there to satisfy older gcc/clang that
don't recognize -Wno-implicit-fallthrough.

Fixes: #2668

This eliminates read-add-zero-write on counters outside this group.

in VSC_C_main and protect them with the ban-mtx.

reports stuff from it for debugging.