Allow server to directly return the final aggregation result

[Jackie-Jiang]

When there is only one server queried, we can calculate the final aggregation result on the server side, instead of serializing back the intermediate result (which can be huge for distinct_count, percentile etc.).

This PR adds a new query option serverReturnFinalResult to let the server directly return the final aggregation result. As a follow up, we may add a flag to the broker config to let the broker automatically add this query option when it is querying only one server.