implement parallel download sketch
implement parallel download sketch

this is a basic outline of what the parallel download scheme will look like.
Many details have been smoothed over to make it easier to test the core
algorithm. These functions will need to be generalized before they can be used
in the actual download code.

rivine/rivine,mharkus/Sia,MrStrong/Sia,rivine/rivine,mnsl/Sia,mtlynch/Sia,mharkus/Sia,NebulousLabs/Sia,MrStrong/Sia,johnathanhowell/Sia,ericflo/Sia,mtlynch/Sia,ericflo/Sia,mnsl/Sia,ChrisSchinnerl/Sia,NebulousLabs/Sia,kustomzone/Sia,ChrisSchinnerl/Sia,Mingling94/Sia