Calls to Backend should be done in Background

Calls to Backend of and object being deleted or updated should be done in Background. Right now many are done in a callback in the model after_commit or after_delete for example.
This is a problem when it fails because there is no retry, and because everything else in the flow that should be done after this line is not done at all.
For example, if the connection with Backend fails in upgrading a plan, then the plan will be partially upgraded in System but its switches and limits will be the old ones.