WIP: Use per-agent locks and GWeakRefs in callbacks from timeout sources
Work on libnice's bug #1 (closed) in Gitlab. This work is composed of multiple merged parts:
"agent: properly handle NiceAgent ref in callbacks from timeout sources"
Phabricator D1898: https://phabricator.freedesktop.org/D1898
This patch was itself based upon a previous version of the work done in D1900. After the switch of hosting, it got lost.
On top of these, additions to follow some review comments from @ocrete: