Versions bump and Changelog update.

`0.9.7.9+deb7u8` is not available anymore for `apt`. Take
`0.9.7.9+deb7u9` from now on.

David Fifield authored Feb 10, 2020 and Georg Koppen committed May 13, 2020

This brings in changes related to
 #33745 Merge a turbotunnel branch
 #33897 Remove buffering from WebRTCPeer
 #34042 Reduce DataChannelTimeout

The new dependencies kcp-go and smux form the new inner session layer.
There's a patch to kcp-go to remove dependencies required for features
we don't use.

Versions bump and Changelog update.

Add instructions for adding/removing/updating users on
build-sunet-a.torproject.net as well as installing new packages.

We also update the tbb-team role to make it possible to remove users.

While updating the README file we also add a link for more details about
setting up a new nightly build machine.

Patch based on dcf's GO111MODULE idea as outlined in #32027. Thanks!

With our switch to including a date into our nightly build versioning
(#25009) testing patches with test builds got harder. The firefox part
has to be rebuilt every day now as it includes the version number, even
if no code changes occurred. That slows down testing of patches
significantly.

We hardcode "testing" as the test build version number to fix that and
use "20010101010101" for the build date.

Thanks to boklm for help.

This reverts commit 8c408b55.

This reverts commit ac8641d2.