ObservationManager#removeEventListener prone to deadlocks

The contract for ObservationManager#removeEventListener mandates: "A listener may be deregistered while it is being executed. The deregistration method will block until the listener has completed executing."

However a strict implementation of this contract is prone to deadlocks: clients unregistering event listeners need to take care not to hold a lock that is also acquired from the event listener being unregistered as this will lead to a deadlock