Adding progress indication for S3 transfers

Now when downloading files from S3, a line similar to the following will appear at the last line of output:

transfers:  99.32%      --      --  25.23%  43.18%      --  88.32% (overall:  54.12%)

This indicates the progress of each individual transfer (currently up to 6 concurrently) along with an rough indicator of total progress (based on the number of files left to download).