Figure out what we should be doing for each stage to update explain("executionStats") output and update the technical design. 

All the other diagnostic channels in this project (slow query log, profiler, $currentOp) all use CurOp to update and report statistics.

We should investigate how existing per-stage and per-query explain statistics get generated. The goal is to report maxUsedMemBytes for both the entire query and also for each blocking stage.