scheduler: Improve task descriptions in debug logs

scheduler: Improve task descriptions in debug logs

Tasks that were simply defined as ordinary functions (not methods) were
identified by their repr() in the logs, which is only really desirable as a
last resort and for objects (where the __call__() method is the task). Use
the function name instead.

Change-Id: Ie34a3602deb968979118b0961bcddf226cf5983f