Squashed 'external/devlib/' changes from 45aebdaca..ac0c39e31

ac0c39e31 connection: Make BackgroundCommand.wait() return non-None
e6323fc8b connection/bg_cmd: fix missing use of signal
7e2399055 connection: update kill command format
ddaa2f162 connection: Rework TransferManager
1c5412be2 connection: Remove dead code
e0b117675 connection: Cleanup TransferManager callback interface

git-subtree-dir: external/devlib
git-subtree-split: ac0c39e31ae60796f3ea435ddd1e47334dd36bd4