Use async code to download files

[sebasmonia]

Currently to download a file I use a sync call.
As per suggestions received in the emacs subreddit, I could add a small delay accepting output before considering the file (or any output) done.

As a bonus I could make the delay configurable, based on my experience of TFS being in a remote location being really slow.

This would impact regular commands too. Everything would benefit from the change in logic. Right now async commands just run with no end in sight. Most of them complete pretty quickly though.

[sebasmonia]

After the merge of waitforoutput I decided to let the files continue being sync for now.

Async commands run faster but the printing part has a few additional challenges. I think shelves, labels and branching are more important now than making this faster.