Concurrent loading with large waveforms fails

Created by: CJvanDiepen

@sldesnoo-Delft @stephanlphilips The background loading fails, which happens when the waveforms are too large, e.g. when initializing all four channels in the pulse library, with a 1.5 ms total duration, and 1E9 sample rate. By failing I mean that each other new waveform, thus after doing the desired number of repetetitions, results in an output of the AWG channels which seems to be at the level of the compensation stage, but on top of that has voltage spikes repeating every 10 us. @sldesnoo-Delft and I are bug hunting.