Parse the cursor responses and aggregate the metrics from them in AsyncResultsMerger.

Change AsyncResultsMerger::_addBatchToBuffer and storePossibleCursors to store parsed cursor metrics in CurOp::OpDebug. After calls to CursorResponse::parseFromBSONMany/parseFromBSONMany, the metrics should be aggregated into CurOp::Debug.

Extend src/mongo/s/query/async_results_merger_test.cpp for AsyncResultsMerger changes