OnCompleted should be called only after WaitUntilDone completes

[glopesdev]

For output nodes operating in FiniteSamples mode, the workflow blocks waiting for samples to be played on cancellation:

daqmx/src/Bonsai.DAQmx/AnalogOutput.cs

Line 143 in 54f63a5

task.WaitUntilDone();

daqmx/src/Bonsai.DAQmx/DigitalOutput.cs

Line 87 in 54f63a5

task.WaitUntilDone();

While the blocking currently works for both completion and cancellation (which is important), it is awkward that OnComplete is sent before WaitUntilDone. It would be more consistent to either have a DAQmx-provided callback or launch a separate task to call WaitUntilDone in parallel, which could then be awaited both to send the completion event or on cancellation.