Replace IAsyncEventProcessor with IQueueingEventProcessor

After internal tests of the IAsyncEventProcessor approach (see !878 (merged)) in LHCb (see lhcb/LHCb!1939 (merged)), it turned out that an interface based on promises and futures it not adequate to the Gaudi event loop, so we decided to replace it with a queue based interface (see LBCOMP-21).

The new interface IQueueingEventProcessor is simple:

• void push(EventContext&&)
  • possibly blocking function to submit a new event for processing
• std::optional<std::tuple<StatusCode, EventContext>> pop()
  • non-blocking function to get the result of a processed event (the order may not match that of pushed events)
• bool empty() const
  • returns true if there's something in the system (either events being processed or results ready to be popped)