Switch from AppVeyor and Wercker to CirrusCI

[jen20]

Neither AppVeyor or Wercker are doing what we need right now (although Wercker is closer). We intend to move to Cirrus CI, which is a managed service which allows both Windows and Linux containers, and to transition our Linux builds to a distribution with the oldest version of glibc that we care about - CentOS 7.

This will take some effort to get working, so right now, _there is no CI system hooked up for either master or for pull requests.

We are working on the new-project-format branch to get everything going again, and will close out this thread when things are good.

This is related work for #1716, but has wider benefits too.

Until this is complete, no other pull requests are being merged to master unless they are part of an SLA-backed commercial request, where other testing will be carried out.

[jageall]

@jen20 is this issue still valid as we are on pipelines for all platforms now?

[shaan1337]

@jageall thank you, we can close it. This was merged through #1710.