Versions bump and Changelog update.

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.