Remove each progress bar immediately upon completion

[brianaydemir]

The overall effect of this PR is to make it so that the client displays progress bars only for "in progress" and "recently completed" transfers. For large, recursive transfers and syncs, this:

• Prevents noisy scrollback buffers.

• Bounds the number of progress bars being displayed to something closer to the number of worker threads and not the total number of objects being transferred, which in turn reduces the number of progress bars that must be removed from the terminal's display when the client exits.

To achieve this:

1. When each progress bar is created, we configure it to be removed when its "completed" event is triggered.

2. We enable each progress bar to trigger a "completed" event when we set its total size. (Because each progress bar was originally created with a size of zero, they will not trigger a "completed" event otherwise.)

And for good measure:

• I reduced the frequency of updates. Every half-second is more than enough to give a feeling of real-time updates.

• I'm letting the deferred progressCtr.Wait() handle any clean up, rather than Wait()ing on each progress bar one at a time.