BLXVIII-142 fixes to get a BufferedTFGScaler class working in continuous scans when the first channel number is not 0
correctly derive the number of channels to read out
pass the datawriter extender to the sink instead of holding it in the DataPointCompletingDataWriter

so the extender is called in the correct sequence wrt the main datawriter (the 'sink')
improve error message
widen columns
remove unnecessary message
allow for slow response from da.server sometimes