Add worker_death function.
Add worker_death function.

function to unblock Worker.collect function if it is requesting data
from a dead worker

jakirkham/dask,clarkfitzg/dask,jayhetee/dask,vikhyat/dask,dask/dask,chrisbarber/dask,ContinuumIO/dask,PhE/dask,mraspaud/dask,cowlicks/dask,jayhetee/dask,mrocklin/dask,PhE/dask,vikhyat/dask,blaze/dask,dask/dask,jakirkham/dask,ContinuumIO/dask,pombredanne/dask,mrocklin/dask,jcrist/dask,pombredanne/dask,wiso/dask,gameduell/dask,blaze/dask,mikegraham/dask,ssanderson/dask,wiso/dask,simudream/dask,simudream/dask,ssanderson/dask,cpcloud/dask,clarkfitzg/dask,mraspaud/dask,jcrist/dask