This was an old attempt at gracefully unloading channels that still had
pending callbacks. Its implementation was always flawed, and the number
of places where it was used is small enough to replace with
straightforward checks for whether the channel has been unloaded after
an asynchronous operation. Hopefully fixes the stuck 0 user channels
issue.
The `cytube-common` module was created as part of a now-defunct
experiment and since then has just remained a crufty container for a few
utils. Moved the utils to the main repo and removed the dependency.