TODOs

Create a separate module btrfs_util and btrfs specific .py files there:
- btrfs_send_{active/passive}
- btrfs_receive{active/passive}
- see also: setup.cfg > console-scripts
print total transfer speed and last chunk transfer speed
create script entry points similar to cat and tee that are independent of the sending and receiving program (btrfs send/receive) in our case:
- sending side: btrfs send | send-chunks-active root@target
- receiving side: receive-chunks-passive | btrfs receive