Release 2019.10 - Soft feature freeze now effective

Dear RIOTeers,

The soft feature freeze (for high impact PRs) is now effective.

I have advanced the milestone for the designated high impact PRs to 2020.01 [1].

As a gentle reminder: the final feature freeze (for all PRs) is 10 October.

As I write this, there are 70 open PRs tagged for the release. 

About 35 of those are relatively actively being worked on. Let's do what we can
to get those in.

Respond to comments, fixup, push, approve, squash, merge, repeat!