Parallelize NVDS Downloads

[JoshStrobl]

I'd appreciate the addition of parallelized downloads of NVDS CVE XML files to saturate more of my connection and speed up the entire download process. Particularly being able to specify a range from "just trickle me the downloads one-at-a-time" to "give me all the CVE XML thingies at once" (which would be helpful when bouncing between my mobile connection and my 250/150 MBit/s home connection).