Update approach to background transfers #74
Description
Currently we use session state to decide when a transfer is complete. Sometimes this leads to commands hanging because internal session state isn't closed even though transfers have completed.
In general, we should not be relying on any internal state from go-car-mirror to know if a transfer is complete. Decide how best to handle this, be that in kubo-car-mirror or go-car-mirror or both, and then update the current push and pull commands accordingly.