Link a few of the classes to implemented features/bugs in python

This adds comments that associate the classes we have for threading
usage to upstream bugs in python where similar features are being
created (and potentially supported upstream).

When we are able to reduce the number of supported python versions
we can/should try to remove our implementations and move to the ones
that may showup in the python standard library instead.

Change-Id: I7b58380aeb57a58fa3b3c424c9f39de30f44f0e9
Merge "Link a few of the classes to implemented features/bugs in python"