There are two major arguments for this:
- There will be more callbacks (e.g. for lock debugging) and extending the
  parameter list is a lot of work.
- In the jqueue module this allows us to keep per-job or per-opcode variables in
  a separate class. Instead of having to clean up the worker class after
  processing one job, these references will automatically go out of scope.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>