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

[janbuchar]

• 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