Multi soundcard fixes

Here is the working merge of our two solution. Your code is working and my is bypassed. 
I have not removed my code, because I will reuse some it for the next iteration like outlined. 
It has still crackling noise on some (e.g. USB 2.0) setups, but but IMHO this is now merge-able, because it is actually working much better than master. 

Features: 
* delay for master and plf (others are not required, because the signal content differs anyway) 
* SampleUtil::clear
* Remove of down mix buffer