Change how conversion and calibration of down samp is done

Instead of do the conversion and calibration of downsampled channel one after another, now these part of code runs as a different thread (one per channel). If when a new pulses arrive the previous one is not calibrated yet, it will be throw away.

This change should avoid the issue when the LLRF got stuck after a fast interlock. This change should be temporary, as a new calibration procedure should be implemented soon.