rados.py: remove Rados.__del__(); it just causes problems

Recent versions of Python contain a change to thread shutdown that
causes ceph to hang on exit; see http://bugs.python.org/issue21963.
As it turns out, this is relatively easy to avoid by not spawning
threads on exit, as Rados.__del__() will certainly do by calling
shutdown(); I suspect, but haven't proven, that the problem is
that shutdown() tries to start() a threading.Thread() that never
makes it all the way back to signal start().

Also add a PendingReleaseNote and extra doc comments to clarify.

Fixes: #8797
Signed-off-by: Dan Mick <dan.mick@redhat.com>
Call Rados.shutdown() explicitly before exit

This is mostly a demonstration of good behavior, as the resources will
be reclaimed on exit anyway.

Signed-off-by: Dan Mick <dan.mick@redhat.com>