remove shared state between worker_core and master
remove shared state between worker_core and master

We used to share a stack and processing set.  Now the worker_core
leaves this coordination entirely to the master.

dask/distributed,broxtronix/distributed,broxtronix/distributed,amosonn/distributed,mrocklin/distributed,broxtronix/distributed,dask/distributed,mrocklin/distributed,mrocklin/distributed,amosonn/distributed,dask/distributed,dask/distributed,blaze/distributed,amosonn/distributed,blaze/distributed