Implement synchronous mode in decision tracing addon

The decision tracing addon, in the current implementation, processes events in a separate thread to minimize the overload on the critical path to the dispatch of the response to the API caller.

We want to provide to developers using the addon the possibility to switch to a full synchronous mode in case they consider tracing itself critial and they're willing to accept a possible increase in the API roundtrip time to make sure the events are completely processed before sending out the response.