Strong reference listen function wrapped by "once"

Strong reference listen function wrapped by "once"

Fixed issue in event system where using the ``once=True`` flag with
dynamically generated listener functions would cause event registration of
future events to fail if those listener functions were garbage collected
after they were used, due to an assumption that a listened function is
strongly referenced.  The "once" wrapped is now modified to strongly
reference the inner function persistently, and documentation is updated
that using "once" does not imply automatic de-registration of listener
functions.

Fixes: #4794
Change-Id: I06388083d1633dcc89e8919eb1e51270f966df38