Mass generator source/destination field validation

This adds an upfront check to the accept() method for the Mass Generator
to make sure at least one note in the set has both the source and
destination field in order to generate the MP3s.
Assertion fixes for router call interface

Corrects the key for the throttle sleep parameter and adds an additional
check for making sure that there is a notification callback.
Fix usage of thread_id vs _current_id

The former was supposed to be used from the signal, not the value on the
class (which is sometimes the same, but not always).
Fix for missing arguments in signals

This was happening because the Worker was being deleted before its
signal was passed. This has been fixed now so that the delete is not
permitted until the signal is passed and the callback is executed.