Update schwa.dr.contrib.processing to parallel worker_main.h

`schwa.dr.contrib.processing` is designed to work with a previous dr-dist that relied on ZMQ REQ/REP. A python interface for the new source/sink/control model (as in https://github.com/schwa-lab/libschwa/blob/develop/src/lib/schwa/dr-dist/worker_main.h) should be adopted, either in the existing `schwa.dr.contrib.processing` or a new framework.
