Normalize task args & kwargs for uniqueness/locking

[thomasst]

As @wojcikstefan pointed out, a task t(x) would have a different ID used for uniqueness and locking depending on whether it was used queued as t(x=1) or t(1), since the arg would either be in args or in kwargs.

This applies to unique and lock_key (see e.g. https://github.com/closeio/tasktiger/blob/master/tasktiger/worker.py#L820-L825).

We should find a way to normalize the task lock IDs.