tasks: Fix and simplify implementation

Recent changes in swh-scheduler add new parameters to the celery tasks
produced from swh.scheduler.model.ListedOrigin instances.

Those new parameters were not properly handled in the celery tasks
implementation for svn loading but also in some svn loader ones.

So ensure to handle any new parameters by not hardcoding the expected
ones in task signatures but also allowing any extra keyword parameters
in loader constructors.

Also remove explicit setting of visit_date if none has been provided
as task parameter as it is already handled in loader constructor.

Related to T4187