kill workers on shutdown
kill workers on shutdown

Fixes #1567

ChrisSchinnerl/Sia,NebulousLabs/Sia,ericflo/Sia,NebulousLabs/Sia,ChrisSchinnerl/Sia,mtlynch/Sia,mharkus/Sia,MrStrong/Sia,mharkus/Sia,mtlynch/Sia,MrStrong/Sia,ericflo/Sia