Add TrackingMap to make Arc/Weaks used more robust

[RitaMAllenCA]

In some places in the node we currently shuffle Arcs and Weaks around, with manual cleanup. This functionality, if not replaced, should be replace with a tested data structure that tracks these things by itself and ideally removes the Weak on Drop, or garbage collects them when prompted.

[marc-casperlabs]

This has been made obsolete with the addition of the new connection manager.