Killing obsolete Jenkins builds

[Norbert Thiebaud]

>
> Norbert, there is clearly a terminology issue in our communication.

:-) quite possible indeed.

>
> I'm suggesting that for a given Change-Id we stop/cancel any ongoing or
> queued builds for that Change-Id before starting a new build.

cancelling build is a tricky/fragile business, especially with the
Windows build, due to the contortion we have to make to be able to
build.

The cost of doing a build like that, especially the second one which
presumabbly would be very similar to the first one, is quite small on
Linux and Mac, due to ccache. we are talking about 5 minute or so each.
Windows gerrit build is pretty much a fix cost of 30 minutes (when
running on tb72/tb73) and a bit more when running on tb68
There is also the issue that sometimes make/cppuunit get stuck which
make the build cost 3 hours.

Norbert