Synchronize `Actor.charge` calls to prevent race conditions in `Actor.push_data`

- calls to `ChargingManager.charge` should be safe because the "critical section" does not yield
- however, when calling PPE-aware `Actor.push_data` there might be some interleaving of the `push_data` and `charge` API calls that may lead to unintended behavior
- it would probably be best to hold all `Actor.charge` calls if there is an `Actor.push_data` in progress
