Add more parallelism for chunk fetches
Add more parallelism for chunk fetches

This results in about 2x the throughput in my testing.  It does spawn
four short-lived goroutines for every chunk that's read, but that
doesn't seem too bad.  It could be optimized into a pool of goroutines
or reusing the worker threads, if its ever necessary.

asjoyner/shade,asjoyner/shade