Implement reusing lanes once they are finished

[Dtenwolde]

Related to cwida/duckpgq-extension#30

Once a lane is finished it currently sits idle until all lanes have been completed. It could be an improvement to reuse lanes as soon as they are finished Need to keep track of which iteration every lane is in. Can be done using a std::vector<int16_t> iteration(LANE_LIMIT, 0); that increments every iteration. Once a destination has been found, the iteration for that lane is reset to 0 and is refilled with the next source-destination pair.