…2649)

Fix incompatibilities identified when running adhoc runs on the new
custom win2019 image.
After merging this, it should be possible to switch to the new image
without breaking any tests.

- for most fixes I added a comment that explains why they're necessary.
- the new image won't have VS2015 installed, so I'm switching the protoc
artifact build to VS2017

This PR will need to be backported to older release branches to ensure
the windows tests continue working on those branches as well (IMHO I
haven't made any changes that would be difficult to backport and I tried
to keeps the diff as small as possible to avoid issues when
backporting).

After we switch to the new image (and all the windows tests are green),
we can incrementally move the builds that are still using VS2017 to
VS2019.