SignalBase: Speed up conversion and prevent leaving samples unconverted

SignalBase: Speed up conversion and prevent leaving samples unconverted

Unconverted samples can happen due to a race condition: being notified
of new samples coming in is ignored when the thread is already executing.
If these notifications however were the last ones sent out because the
segment was completed right after, the currently ongoing conversion
will finish but not be restarted with the final sample range.

With this patch, this is prevented.