Add a --threads option
To increase performance, we can add multithreaded HTTP requests toward Gitlab. This will, however, increase traffic and may be unwanted.
Proposal: Add a --threads option which allows the user to set the number of HTTP requests to perform at any one time. Default this option to a value of 1, causing one thread to be used for all requests.
This way, the user can then choose exactly how much traffic they'd like to hog.
Note With --threads, the order of catching exceptions and recovering from them
changes with a multithreaded flow, as we can no longer simply treat it sequentially.
We will need to modify the way we deal with exceptions during command_import.