add (dumb) throttling implementation

this MR:

• prevents dl from falling down for files larger than 100MB by implementing a dumb request throttling strategy (to wit: only allow a maximum of 32 requests to be in flight at any given time by determining the number of pieces to request as N/32, where N is the size of the file)
• updates the README to (1) describe this strategy and (2) provide updated benchmarks proving that the new solution works