Currently when we need to update a CrOS DUT, we need to call devserver to stage artifacts, call DUT to fetch artifacts from devserver, and let DUT execute auto-update. The process is too complicated to debug, and easily down due to network congestion/flake. 

We want to change the workflow to: let devserver call DUT to update. The DUT will never pull devserver to update. Instead, devserver will push the payload to DUT and call it to update. 

The advantages include:
1. Reduce errors caused by network congestion/flake: 
remove the two most flake rpcs: cautotest calls DUT to auto-update, and DUT pulls devserver to transfer payload. Change them with a much reliable rpc: devserver pushes payload to DUT, and let DUT auto-update.

2. Easy to debug. Move every steps, such as stage, transfer payloads, and execute auto-update to devserver, and log relative debugging errors.

3. Leave room to modify devserver's form. 

4. Leave room to integrate the auto-update logics for all devices and images to this framework.