aggregator: Wake up src pad task on reconfigure event

For instance, we want to resend the latency query, in case the upstream latency has changed.

Probably needs !356 first. Otherwise, this scenario breaks:

1. fallbackswitch is running
2. something upstream of the fallbackswitch sinkpad is unlinked
3. fallbackswitch receives reconfigure event and sends latency query upstream
4. latency query fails because upstream is unlinked
5. aggregator in fallbackswitch waits forever for the latency query, never switches to fallback